If you're playing 1-3-1, your PF also needs to play some outside defense, so it's important he's good at that. Diagrams of the defenses are below. Disregard the periods - they're there so you can see where the players are positioned on the court.
1-3-1
... *
* . * . *
... *
2-3
.... *
* ...... *
.. * . *