My assumption is that in a 1-3-1 defense, the players are postioned as follows:
PG at top of perimeter,
SG near free throw area
SF & PF outside of the box
C near the basket
Of course, not really too sure, lol.
It could be like this:
..............O.............
.............................
..............C.............
.............................
SG..........PF..........SF
.............................
.............PG.............