Which 3 styles of martial arts are best for MMA?

Stand up: ?
Takedowns: ?
Ground game: ?
Thanks!

Stand up: Muay Thai is probably the best because it goes over all the stand up aspects allowed in mma, low kicks, body kicks, head kicks, punch combos, knees, and thai clinch. But its best to mix it up and to have variety so I suggest a combination of Muay Thai, Boxing, and Sanshou.
Take downs: your best bet is freestyle wrestling because it gos over more aspects than greco-roman. No gi Judo is great also because it can catch opponents by surprise and its hard to defend against, but it is very hard to master. But like always mix it up, between freestyle, Greco roman, and no gi judo.
Ground game: You can't really beat bjj with anything nowadays, the only thing that can beat good bjj when your on the ground is bjj that is better, so Brazilian jiu jitsu is best on the ground. But since every one does bjj it doesn't hurt to have things like Sambo and Catch wrestling on the ground either, because every one does bjj it doesn't hurt to add in something new after you mastered the necessary things.

My opinion

Stand up: Muaythai, san shou, or a combo of Chinese gungfu and ch. boxing
Takedowns: a shooto/wrestling hybrid (would have said bjj before some of the best Wrestling pros learned how to best it in the last ten years or so)
Ground: (same as takedowns)

Stand up: Boxing
Takedowns: Judo
Ground game: Bjj

I'd suggest,

Best wishes ***

Stand up- muay thai

Takedowns- wrestling or judo

Ground game-BJJ.

Though just as good and what MMA pros do is
1. treat MMA as the main martial art itself, where you will learn most of everything.

2.Then specialise in some extra boxing for stand up.

3.And extra wrestling for your takedowns.

The reason for boxing being a better choice for them than muay thai is mastery of the hands,the hardest and most important skill for stand up is better taught by pure boxing. knees and elbows and kicks can be easily enough taught at the MMA class itself.

The main advantage of doing the extra wrestling training is not to learn any new takedowns, (since its easily enough taught at MMA), but just to give you a chance to train with pure wrestlers, who are the best at it and will sharpen you more.

Stand up: Muy Thai
Take downs: Judo, & Wrestling
Ground Game: Brazilian Jujitsu

Stand up- muay thai

Takedowns- wrestling or judo

Ground game-BJJ.

Though just as good and what MMA pros do is
1. treat MMA as the main martial art itself, where you will learn most of everything.

2.Then specialise in some extra boxing for stand up.

3.And extra wrestling for your takedowns.

The reason for boxing being a better choice for them than muay thai is mastery of the hands,the hardest and most important skill for stand up is better taught by pure boxing. knees and elbows and kicks can be easily enough taught at the MMA class itself.

The main advantage of doing the extra wrestling training is not to learn any new takedowns, (since its easily enough taught at MMA), but just to give you a chance to train with pure wrestlers, who are the best at it and will sharpen you more.