These are the differences I encountered with Python 3.
There are many new libraries. Some deprecated libraries are replaced with better alternatives.
Python 3 has unicode support for strings. (This is good and bad, as it can require more work porting some Python 2 programs).
The print statement becomes a function, i.e. you must use curly brackets with print statements.
There is no "xrange" statement, and "range" behaves like Python 2's "xrange."
For Pythonista 3, if you push the "run" button and hold it, an option appears that will let you select to run the program with Python 2. Alternatively, if the first line of the program is:
#!/usr/bin/python2
then you can just push the "run" button, and the program will run under Python 2.
You probably don't need Python 3 for the problems you're solving, however, as noted, Python 2 is not advancing anymore, so you might want to know Python 3 for the future.