Can you be put on 3 years probation for a DWI?

Yes. What were the circumstances?

If this is a second offense then yes. If this is a first offense, then it seems harsh but there could be extenuating circumstances that the Judge decided to give that much probation.

Yes, you can, each state in the US has its own rules and even each judge his own ideas of if they give the max allowed or not

I was a probation officer in MD for 7 years and I once had a person on 5 years probation for DUI. However, this person killed another person. Surprisingly, they never spent one day in jail!! As was mentioned above, the laws vary from state to state.