You limit the number of picked instances with conditions on the left and apply actions to those picked on the right. If you don't pick anything with a condition on the left then it applies the action to all instances. If you used something like on object touched, do an action then it picks the one you touched. If you said where object.var=3, do an action then it does the action to all instances where var=3.
But that isn't really your issue here, you need to relate A to the B and C that are overlapping and the only way I can think of based on info given is to have instance variable identifier where related A,B and C have the same number and when you pick which B overlaps C, you pick the A which has the same variable number attached.
If that doesn't help then please share some more information about your game, maybe there is a better way to get the logic you want.