While they may offer compensation, I'm pretty sure they're not required to. The contract of carriage (your ticket) basically says they'll get you from point A to point B, but nothing about a specific time.
That being said, I don't think it would hurt to ask, but think about what you're asking for. Maybe you want some kind of credit to pay for a meal at the airport while you wait. However, if you do decide to leave the gate area, make sure that plane isn't leaving for 3 1/2 hours. It's possible they come up with another solution, and the planned 3 1/2 hour wait turns into 1 hour.