Things to consider when having a single enemy have multiple actions: how much can they do without any possible player input. Can they 100->0 a player with 3 attacks? Can they get into range, attack, move out of range, so on their turns the melee players have to dash and never get to attack? Legendary actions split the bosses turn into multiple initiative steps to allow players to react in between actions.

It could be a cool fight, if the boss gets to do one thing after every player turn (either move, action, or bonus action). It completely changes the fight dynamic and might run into problems for actions that also use movement etc but it feels more interactive than getting hit by an enemy that always runs out of your movent range after every attack.

If it's a recurring villain, I would give them legendary actions which usually do exactly what you describe: it allows to make another attack, or use a feature. It is also not in the same turn, so it would even enhance their sneak attack (assuming they have that, too).

I don't think giving them a feature that says "Multiattack: Master Thief makes 3 attacks. Alternatively it can replace any number of those attacks with Cunning Action or Special Action." comes too long or cumbersome. Assuming I understood you right and that is what you are looking to do.

Legendary Actions, as has already been suggested is also a good call.

Or you can just give them 3 actions, that works. Because you are the DM you just have to notice and make a call when an unintended interaction comes up and avoid abusing it. Only thing I can think of is abilities where "as an action" is supposed to translate to "as their turn". Things like breaking out of an Entangle spell. You should probably avoid just doing that multiple times to not make the person who cast the spell feel like it was useless due to this unconventional situation.

If you want something mechanically simpler, it sounds like giving your NPC a permanently hasted condition would accomplish most of what you're asking. Two actions, extra movement, plus the Thief bonus actions would allow for three attacks per turn (if dual wielding).

Other answers recommending legendary actions are great too. But if you wanted your NPC to be more "vanilla," the perma-haste might work.