Sitting in the drivers seat, #3 would be on the passenger side closest to the firewall.
In drivers seat picture it like this
Drivers seat 4 2 Front of Vehicle-->
Passsenger seat 3 1
You can locate the #3 cylinder on a wrx is located in the back on the passengers side. Its so funny that I have also been looking for that on my previous car, but now im happy with my vw beetle accessories in my car.