The framework is compatible with both.
However, your own code may only be compatible with one or the other - that's up to you.
Once you get to know what the differences are, you can scan the code pretty easily yourself. Still, the 2to3 is helpful if you have lots of code.
One simplistic approach is simply to make a copy of your app, try it under python 3, and fix what is broken.
Important: anyone making such a conversion is being increasingly encouraged to also consider porting it to py4web. This is a steeper learning curve, but it may be a better long term solution.