It may be common among certain people and groups, but I personally have never heard anyone I know speak like that. In spoken English you often hear the "do" as part of the "what" and the "you" pronounced with the same vowel sound as "what" so it sounds like "Whataya think?"

Hello, My name is maris, which is really nice to get on this site, I took interest in you after reading your profile at (www.italki.com). ([email protected]) I believe age and distance has nothing to do with love, so just drop me an email through my private e-mail address ([email protected]), so that we can communicate better and I send you my images. Thank you. maris.

Colloquially you can say "whatcha think?", kind of a linguistic short cut for "what ya think?" (what do you think?)

"What do you think?" is correct.

There are dialects of English that would say "What you think?" However, unless you are trying to associate yourself with this particular dialect, I wouldn't use it. The standard way of saying this is "What do you think?" If you are trying to learn Standard English, then I'd suggest the latter. It all depends on which dialect you are trying to learn. Good luck!