(sailboat); A,B,C... - mission types
(1) = A
(2) = B
(2)+(3) = C
(3)+(1) = C
You have also "new kind" of mission for next variants:
(3) = D
(1)+(2)+(3) = E
(1)+(2) = [A,B] = F
My proposition:
chance of mission A = chance of mission B = chance of mission C
(place 3 missions in dynamic group with minimum/maximum =1)
and if:
A then create (1)
B then create (2)
C then create (1a)+(3a) or (2b)+(3b)