What you need to find out is whether your middle P90 pickup is in reverse magnetic polarity and reverse wounded to the other two pickups. This is often referred to as RWRP and can be ordered from most pickup manufacturers with this specification.
Next you need to learn about pickups connected in series or in parallel.
Assuming the middle pickup is RWRP to the neck and bridge pickups, you will have two more pickup selection choices. If you use a Strat type 5 way switch, these are the switching options available :-
Position 1 - Neck pickup on.
Position 2 - Neck and middle pickups on in parallel (hum-canceling)
Position 3 - Middle pickup on.
Position 4 - Middle and bridge pickups on in parallel (hum-canceling)
Position 5 - Bridge pickup on.
Now if you add a switch to active the middle pickup to be on at all positions,
Position 1 - Middle pickup joins up to Neck pickup in series making this position work and sound like a humbucker for a fat and creamy tone without hum.
Position 5 - Middle pickup joins up to Bridge pickup in series making this position work and sound like a humbucker for a fat and biting tone without hum.
P90 pickups sound particularly sweet in the above 2 positions with the added versatility of emulating humbucker tones.