You should create a new global or local number type variable (let's just call it "wheretospawn") and at the event:
Object A destroyed: System -> Set Value (wheretospawn) -> int(random(3))
Then insert 3 sub events:
System -> Compare variable (wheretospawn) -> =0
System -> Compare variable (wheretospawn) -> =1
System -> Compare variable (wheretospawn) -> =2
Then for each of these sub-events you add the Create object actions for object A, B, and C.