Credit to apple1417 for his research (code finding, spreadsheet work).
Here are the 9 sets of spawns that can appear. Note that these sets only apply to maps with 3 potential spawn points, not to all maps!
Sets A, E, and I are twice as likely to appear.
Some sets are impossible to get based on your Paint 4 set. Select a Paint 4 set to see what spawns are possible:
A1 A4 B1 B2 B3
A 
B 
C 
D 
E 
F 
G 
H 
I