what would be the best language to write an aimbot for a 3rd person shooter?
Tl;Dr: Just don't do it!
This would depend on the language the 3rd person shooter is in. Take roblox for instance. Sure, you could probably find a way to inject code as assembly and interfere with the data sent to the game, but it would be better to write it in Lua (because that's what Roblox uses). It depends on what the game is coded in, really.
Also, might I remind you making an aimbot is probably against any game's ToS (Terms of Service) meaning you could get banned, and in some cases sued.
Just don't do it. Plus, you would have to find a way to "interfere" with the data sent (game/platform dependant). Most of the time there are measures taken in place to prevent this kind of stuff, and most of the time you can't just "change the entire code of the game (or even just a little)". From what I see, in Roblox you add to the code, not change it (which means that anti-hacks might work really well).
I don't know how Roblox works, but for applications it will be more difficult, unless it has its own scripting features
There are two answers here:
1. Are you writing an aimbot for your own game? Then use the language you're using in your game.
Writing aimbots is against most TOS's in games
It is strongly advised against to write aimbots for multiplayer games, as this is typically a violation of their terms of service, and will get you banned.