Like the title said, I usually don't play Rogue and I'm just getting into it, but I almost spilled my coffee when I realized that Rogues can basically cast 3 spells per turn: 1 per action, 1 per bonus action, and 1 per reaction.

And oh boy this is SO BUSTED. There are so many combinations, and I'm really excited to try them :)
Command: Flee => Dissonant Whispers.
Fireball => Hypnotic Pattern.
Pyrotechnics: Smoke => Wall of Force.
Scatter => Cone of Cold / Spirit Guardian / any AoE spell.
Plant Growth => Web.
Beacon of Hope => Mass Healing Word.

There are so many! :D

Edit: never mind!

Nah man, Thieves are OP :D

Level 13: Use Magic Device

You’ve learned how to maximize use of magic items, granting you the following benefits.

Attunement. You can attune to up to four magic items at once.

Charges. Whenever you use a magic item property that expends charges, roll 1d6. On a roll of 6, you use the property without expending the charges.

Scrolls. You can use any Spell Scroll, using Intelligence as your spellcasting ability for the spell. If the spell is a cantrip or a level 1 spell, you can cast it reliably. If the scroll contains a higher-level spell, you must first succeed on an Intelligence (Arcana) check (DC 10 plus the spell’s level). On a successful check, you cast the spell from the scroll. On a failed check, the scroll disintegrates.

Also, if I'm playing this, I'm buying a Headband of Intellect, and I take proficiency in Arcana (I'm thinking through the Skilled Origin Feat). With Reliable Talent, I'll never fail that Arcana check :D

Oh you’re absolutely right, I totally missed that.

Aren’t you still going to run into the general rule for spellcasting of only one leveled spell per turn? 

Quote from Xalthu >>

Aren’t you still going to run into the general rule for spellcasting of only one leveled spell per turn? 

The rule (in 2024, anyway) is that you can only cast one spell with a spell slot per turn. Scrolls don't use spell slots.

Quote from wagnarokkr >>

Quote from Xalthu >>

Aren’t you still going to run into the general rule for spellcasting of only one leveled spell per turn? 

The rule (in 2024, anyway) is that you can only cast one spell with a spell slot per turn. Scrolls don't use spell slots.

 Yes. I realized that about 10 minutes after I posted this. But thanks for the correction.

Not saying you can't but.

Command: Flee => Dissonant Whispers.                                            100gp
Fireball => Hypnotic Pattern.                                                                 600 gp
Pyrotechnics: Smoke => Wall of Force.                                                3200 gp
Scatter => Cone of Cold / Spirit Guardian / any AoE spell.             20,000+gp
Plant Growth => Web.                                                                              500gp
Beacon of Hope => Mass Healing Word.                                              600gp

And that is assuming you can come across them. If a party member crafts them 1/2 that. 

now while it will frequently require attunement wands/staves can come in handy here. as like a wand of fireballs does not go away when used, so its increased cost of 4,000 you can get a lot of value out of, even if its just 2 fireballs in a round a couple fights a day. 

If you're level 13, money rarely is a problem :)

If you say so. But that kind of money has been a problem in almost every 5e campaign I have seen. But every table is different. 

Beyond the money issue (which is significant in a non-Monty Haul campaign), you run afoul of the problem that storebought scrolls have relatively low hit/DC. So anything that isn't buff/utility tends not to be all that worthwhile in scroll form. You can get around this problem by having a party member scribe the scrolls (which is still expensive). However, then you've got to ask yourself whether you're better casting as a Bonus Action or forcing Disadvantage on the save via Arcane Trickster. For most offensive spells, the latter is normally superior because Rogues already have an overloaded Bonus Action.

Thieves at level 13 use their spellcasting ability modifiers for spell scrolls:

Scrolls. You can use any Spell Scroll, using Intelligence as your spellcasting ability for the spell. If the spell is a cantrip or a level 1 spell, you can cast it reliably. If the scroll contains a higher-level spell, you must first succeed on an Intelligence (Arcana) check (DC 10 plus the spell’s level). On a successful check, you cast the spell from the scroll. On a failed check, the scroll disintegrates.

And with expertise in Arcana, you'll never fail the check.

The "using intelligence as your spellcasting ability" phrase only means that if the spell description contains some feature reliant on spellcasting ability (such as Dispel Magic), the Rogue's Intelligence would be used. Since spell scrolls don't use spellcasting ability to calculate hit/DC (they're fixed upon creation of the spell scroll), your spellcasting ability doesn't matter for their hit/DC.

Ordinarily, the only way you can use a Scroll is if it appears on your spell list - which tells you what the spellcasting ability associated with that spell will be. However, you still use the default hit/DC regardless of what your own personal stat will be. Nothing about the Thief's Scroll Use ability changes this rule - it merely identifies what spellcasting ability you should use if it's called upon (which it normally isn't) since Thief Rogues aren't using Scrolls of spells that appear on their list.