I'm sure SPSS has some way to do this test, but I'll attach an ugly-looking Excel worksheet that shows an example.
One note: if you do 3 tests (proportion 1 vs 2, 1 vs 3, and 2 vs 3), then instead of using the usual .05 "p-value hurdle", you should adjust it to .05/3 = .0167. This is done to account for the increased likelihood of making an error due to doing more tests.
Again, this is just my non-expert opinion. Hopefully someone can confirm/correct.
Good luck on your thesis, hope this helps.