1 - This is fine. I think see here is an example of the futurate. It's asking what you will do if you see X in the future. EDIT - as @Lambie points out, maybe "futurate" is not the appropriate term here. It's a hypothetical about something that could happen in the future.
2 - This is what I would go with for something that could happen in the present. Maybe it seems counterintuitive to an English learner, but even though this sounds like the past tense, it's not really talking about the past. You could say:
If you saw X right now, what would you do?
If you did want to talk about a possibility in the past, you would probably use the past perfect:
If you had seen X (yesterday, e.g.), what would you have done?
3 - This doesn't sound grammatical; you can't mix the present tense and conditional mood in this case. In particular, I think the combination of present-tense verb + past-tense modal verb (could, should, would) does not work. @Jasper came up with a good counterexample to my original statement that the present tense and conditional mood don't ever mix - that's not true. I think it is fair to say that the verbs could, should, and would, when used to indicate hypothetical situations, can only be used with past-tense verbs. (Happy to revise my answer if someone can refute this.)