Step 1. After installing geany you want to go into the terminal and enter this:
sudo <text editor here> /usr/share/geany/filetypes.python
Doing so will ask for your password and subsequently bring you to a text file for geany's python settings.
Step 2. Go to the bottom of the file and comment out the bottom two lines - Just add a # in front of them, this means if you want to go back the the 2.x compiler you can do so by following these instructions again.
Step 3. Now copy paste the following two lines into the bottom of the document:
compiler=python3 -c "import py_compile; py_compile.compile('%f')" run_cmd=python3 "%f"
Step 4. After you save it the next time you open your python3 script in Geany you will be able to run (f5) and use it as python3 (Assuming that is what you code in)
To revert back to 2.x, all you do is go to that file again and un-comment the lines regarding 2.x and comment out the 3.x lines.