3 hours to change aircraft in transit, is that enough?

depends on the time of day and the price of fuel

3 hrs is usually enough time for domestic flight. If you are traveling international then you have to clear customs/immigration and collect baggage before next flight segment. In this situation you might need more time. Do you have to re-check baggage? Also, you will have to go through TSA screening. I would suggest checking with your airline to see what the process will entail for planning time accordingly.

Depends