should be pretty easy though,
you could have three global variables combo1, combo2 and combo3
on object1 selected add 1 to combo 1
on object2 selected add 1 to combo 1
on object4 selected add 1 to combo 2
if combo1 = 3 set bonus
if combo2 = 3 set bonus
All you'd have to do is make sure the objects can't be selected more than once and/or subtract 1 when deselected.