Legendary Actions and CR

Can I make a monster more then the max three legendary Actions, and does it effect the CR, cuz the way I'm thinking is that those are just attack names, also can I have monster have more then one replacement spellcasting ability like have both Charisma and Intelligence as spellcasting ability at the same time, and finally does the legendary Actions only involve the At Will spells or can it also include the 1 or 2 /day spells as well

-Some monsters already have more than 3 Legendary Actions. This is often attached to being in their lair but there's no rule requiring that they must cap at 3.
-If legendary actions are increasing the expected damage per round or impactful effects, it should be calculated into CR. Otherwise no.
-Using multiple stats at once like that is a rare thing and generally not used for spellcasting modifiers; use this sparingly, but there's not a rule that monsters can't do this. If doing this in a way that only works as Legendary Reactions, just specify that.
-Any spells can be part of the Legendary Actions, including spells that aren't otherwise part of their spellcasting feature; if it uses their spellcasting feature, just specify as such. Also might be worth limiting to 1/day or so.

TL;DR In order: You can if you want, but it's unorthodox. Yes, it would raise the CR. I don't fully grasp what you want, but if I understand you correctly, you CAN do whatever you want, but it's unusual. Legendary actions involve... whatever Legendary Actions are listed.

Wordy answers now: You can have more Legendary Actions if you want, but it's unorthodox. This is because Legendary Actions are supposed to counter player action economy by essentially giving the monster a "mini-turn" on the players' turns. Adding additional Legendary Actions could help counter a party with more players, summons, or a lot of extra actions from things like Action Surge, but the more traditional approach is to include additional, supplemental enemies, or to add challenges prior to the Legendary encounter which drain enough player resources to balance the books. Additional Legendary Actions would increase the monster's CR, because it would increase their action economy, making them considerably more dangerous. How much it would affect the CR is up to a lot of factors: how strong the monster is to begin with, how many Legendary Actions you're adding, and what Legendary Actions are available. For Instance, an Aboleth getting additional Legendary Actions would be less impactful than, say a Lich, both because the former is weaker, and because its action choices are far less impactful to the characters.

As to your question about multiple spellcasting abilities, I assume you mean can they stack spellcasting abilities (for instance, a monster adding both its Intelligence and Charisma bonuses to its spellcasting ability modifier), rather than can they have spells with different spellcasting abilities (for instance, a monster mimicking a multiclass Druid/Sorcerer who might have Charisma as the spellcasting ability modifier for some spells and Wisdom as the spellcasting ability modifier for others). This typically isn't done, largely because it's unnecessary and redundant. If you want the creature to have a higher spellcasting ability modifier, you can just increase its spellcasting ability score. If you want to have a high spellcasting ability modifier without increasing its spellcasting ability score... just... give it one. You can say they've got a magic stone in their eyeball that augments their spells, or they have a natural affinity for fire spells or whatever. It's your monster, you don't need to rewrite the rules to make one monster stronger.

Last, you can make anything you want a Legendary Action. As a general rule, spells that aren't at-will don't get included because it complicates the actions, and because Legendary Actions are typically the equivalent of just making a basic attack. There are exceptions with higher-powered opponents, but generally, you don't want to have a Legendary Action that is equal in power to taking a full action.

Ultimately, you're the DM, so you can make whatever you want and adjust the rules any way you see fit, but the creature creation guidelines exist for a reason and have been long battle-tested. I find that, if I'm feeling an urge to stretch outside the guidelines, I'm often overreaching in a way that will ultimately backfire. Anybody with any experience will tell you that tailoring a solo BBEG to throw at your party is a bad idea. If you want extra actions, add extra enemies. It gives your players more targets, gives you more actions, and spreads out the risk that someone will crit and absolutely obliterate your entire fight in one swing.

You're the DM, you can homebrew whatever you want.

For the multiple spellcasting ability, no, I mean use charisma and and intelligence for certain spells, like a Adult/Ancient Red Dragon using charisma for command and scotching ray, and intelligence for Telekinesis and Invisible Mage Hand.

More then three Legendary Actions: still on the red dragon, I want to add to more Actions, can I do that

But everything else was perfect info thx

Quote from RWinnie >>

TL;DR In order: You can if you want, but it's unorthodox. Yes, it would raise the CR. I don't fully grasp what you want, but if I understand you correctly, you CAN do whatever you want, but it's unusual. Legendary actions involve... whatever Legendary Actions are listed.

Wordy answers now: You can have more Legendary Actions if you want, but it's unorthodox. This is because Legendary Actions are supposed to counter player action economy by essentially giving the monster a "mini-turn" on the players' turns. Adding additional Legendary Actions could help counter a party with more players, summons, or a lot of extra actions from things like Action Surge, but the more traditional approach is to include additional, supplemental enemies, or to add challenges prior to the Legendary encounter which drain enough player resources to balance the books. Additional Legendary Actions would increase the monster's CR, because it would increase their action economy, making them considerably more dangerous. How much it would affect the CR is up to a lot of factors: how strong the monster is to begin with, how many Legendary Actions you're adding, and what Legendary Actions are available. For Instance, an Aboleth getting additional Legendary Actions would be less impactful than, say a Lich, both because the former is weaker, and because its action choices are far less impactful to the characters.

As to your question about multiple spellcasting abilities, I assume you mean can they stack spellcasting abilities (for instance, a monster adding both its Intelligence and Charisma bonuses to its spellcasting ability modifier), rather than can they have spells with different spellcasting abilities (for instance, a monster mimicking a multiclass Druid/Sorcerer who might have Charisma as the spellcasting ability modifier for some spells and Wisdom as the spellcasting ability modifier for others). This typically isn't done, largely because it's unnecessary and redundant. If you want the creature to have a higher spellcasting ability modifier, you can just increase its spellcasting ability score. If you want to have a high spellcasting ability modifier without increasing its spellcasting ability score... just... give it one. You can say they've got a magic stone in their eyeball that augments their spells, or they have a natural affinity for fire spells or whatever. It's your monster, you don't need to rewrite the rules to make one monster stronger.

Last, you can make anything you want a Legendary Action. As a general rule, spells that aren't at-will don't get included because it complicates the actions, and because Legendary Actions are typically the equivalent of just making a basic attack. There are exceptions with higher-powered opponents, but generally, you don't want to have a Legendary Action that is equal in power to taking a full action.

Ultimately, you're the DM, so you can make whatever you want and adjust the rules any way you see fit, but the creature creation guidelines exist for a reason and have been long battle-tested. I find that, if I'm feeling an urge to stretch outside the guidelines, I'm often overreaching in a way that will ultimately backfire. Anybody with any experience will tell you that tailoring a solo BBEG to throw at your party is a bad idea. If you want extra actions, add extra enemies. It gives your players more targets, gives you more actions, and spreads out the risk that someone will crit and absolutely obliterate your entire fight in one swing.

For the multiple spellcasting ability, no, I mean use charisma and and intelligence for certain spells, like a Adult/Ancient Red Dragon using charisma for command and scotching ray, and intelligence for Telekinesis and Invisible Mage Hand.

More then three Legendary Actions: still on the red dragon, I want to add to more Actions, can I do that

But everything else was perfect info thx