Hey so I ran into a problem in one of my games and am looking for a solution to prevent it from happening again

The short of it was some players were going to sneak up to a young red dragon and attack it during the first round of combat. However they thought they would get surprise on the dragon but I ruled no. There was a fight but I "won" and the dragon is not surprised

Now one of there arguments contradicted the RAW in that if a person came into a battle between enemies as a third party they would get surprise (The condraticition is that as combat was already underway surprise would not occur)

Wha I want to know is can there be a mechanical or other way to explain that enemies or creature already in combat cannot be surprised (My way is to explain that  creature in battle are considered active combatants and active combatants are immune to the surprise condition)

If you have a better idea I would love to hear it

Actually i am going to side with your players and here's why

picture this you are in the middle of a fight with four people and four more show up and are actively trying to stay hidden while you are busy with the other people. are you going to notice all 4 other new people who are actively hiding waiting to jump in. the likelihood is no. you might notice one but then you are splitting your attention even further. the more attention you (or in this case the dragon) is giving to the people it is fighting the less likely you are to notice hidden things. especially if you are fighting for your life. you have to remember dragons are intelligent. they have gold, themselves and possibly children to keep safe

The surprised condition means you can not move or take actions on your first turn of combat and can not take reactions until that turn ends. If a dragon perceives a threat of 3 adventurers (not knowing there are in fact 5) you would expect it to act on its first turn of combat.

If it has failed to notice the adventurers that are sneaking then clearly they have all the advantages of being hidden (e.g .attacks at advantage) but it is less obvious whether they have the advantages of surprising the the dragon by virtue of their presence there are two obvious features that come to mind

1. If the dragon has not had its first turn can it use its reaction is response to an action of one of the sneaking party members
2. Does the dragon count as surprised with regards to special features such as Assassinate

RAW you are either surprised or you are not surprised it doen't relate to individual creatures so I think you are correct. If a creature misty steps and hides to the enemy has no idea where it is and attacks next turn it can still cast shield to block the attack next turn (obviously if it has the spell not talking about the dragon here) so RAI I would also tend to go with you.

ROC if you think the players have a well thought out plan and you want to reward them for their inginuity then sure let assassinate work.

Extra combatants in a fight will not cause the dragon to lose its actions. I would rule this as the dragon cannot react to the PCs during the surprise round but may act normally against whatever it is currently fighting (this is a decent general rule: surprised creatures that are in the process of doing something will continue doing whatever they're doing during the surprise round, rather than doing nothing).

thanks everyone

I think I now have what I need to solve the problem in a fair and accurate way

Quote from Goldkeeper777 >>

Hey so I ran into a problem in one of my games and am looking for a solution to prevent it from happening again

The short of it was some players were going to sneak up to a young red dragon and attack it during the first round of combat. However they thought they would get surprise on the dragon but I ruled no. There was a fight but I "won" and the dragon is not surprised

Now one of there arguments contradicted the RAW in that if a person came into a battle between enemies as a third party they would get surprise (The condraticition is that as combat was already underway surprise would not occur)

Wha I want to know is can there be a mechanical or other way to explain that enemies or creature already in combat cannot be surprised (My way is to explain that  creature in battle are considered active combatants and active combatants are immune to the surprise condition)

If you have a better idea I would love to hear it

 The way to normally determine surprise is straightforward, either the DM determines who might be surprised or rely on Steath vs Perception to figure out.  If you ruled there was no surprise, then the dragon wasn't surprised. 

If a creature enter an ongoing battle, there is no surprise as surprise only ever occur at the begining of combat, having an incidence on your first turn. People engaged in combat has already noticed a threat and can't be caught offguard thereafter.

There is an official ruling in Sage Advice on this  Rules Answers: November 2015 | Dungeons & Dragons (wizards.com)

Does surprise happen outside the initiative order as a special surprise round? No, here’s how surprise works.

The first step of any combat is this: the DM determines whether anyone in the combat is surprised (reread “Combat Step by Step” on page 189 of the Player’s Handbook). This determination happens only once during a fight and only at the beginning. In other words, once a fight starts, you can’t be surprised again, although a hidden foe can still gain the normal benefits from being unseen (see “Unseen Attackers and Targets” on page 194 of the Player’s Handbook).

To be surprised, you must be caught off guard, usually because you failed to notice foes being stealthy or you were startled by an enemy with a special ability, such as the gelatinous cube’s Transparent trait, that makes it exceptionally surprising. You can be surprised even if your companions aren’t, and you aren’t surprised if even one of your foes fails to catch you unawares.

If anyone is surprised, no actions are taken yet. First, initiative is rolled as normal. Then, the first round of combat starts, and the unsurprised combatants act in initiative order. A surprised creature can’t move or take an action or a reaction until its first first turn ends (remember that being unable to take an action also means you can’t take a bonus action). In effect, a surprised creature skips its first turn in a fight. Once that turn ends, the creature is no longer surprised.

In short, activity in a combat is always ordered by initiative, whether or not someone is surprised, and after the first found of combat has passed, surprise is no longer a factor. You can still try to hide from your foes and gain the benefits conferred by being hidden, but you don’t deprive your foes of their turns when you do so.