O2 Sensors ? - 1996 Sierra 4.3L K1500

I am replacing the catalytic convertor and all the sensors. I want to be sure what type of O2 sensors I actaully need. The reason I ask is when looking for parts they all state "2 required" whether its a "pre" or "after" cat. Am I correct that I would order 3 pre and only 1 after?
Thanks for any assitance !

Not sure on the 4.3. My 97 5.7 has 2 fore, and 2 aft, i belive. Just look under your rig and see. what ever you got, replace it.

2 pre and 2 after. one before and after each convertor. the after pair is just for evaluating convertor performance. the pre sensors actually help control the fuel mixture.