I have two different type o seat in my shower faucet. Can someone explain to me why? One is round. One is hex. The center one is hex. Hot and cold is round.
Your seats are not replaceable. Use a re-seating tool to grind off any imperfections on the seats and replace the stems. Very common situation in laundry tub faucets, not common in other faucets.
Picture of the stem you took out.
I'll bet it's a Price Pfister ceramic disc type. the hot and cold stems do not have seat. They have a seal on the end of the stem assembly.
The diverter has a seat because that is the standard type stem.
Your seats are not replaceable. Use a re-seating tool to grind off any imperfections on the seats and replace the cartridges. Very common situation in laundry tub faucets, not common in other faucets.