https://www.askmehelpdesk.com/attachments/plumbing/29168d1266095902-how-change-shower-faucet-washer-tub-shower-3-knob-valve.jpg

It appears that your diverter in the center that has the leak and here is a video that shows how to fix it.

https://www.youtube.com/watch?v=cmYPRNwwpeM

Let me know if it helps.

It looks like old American Standard tub / shower faucet. Yes, they have washers and seats readily available for this faucet. You may also replace packing seal at the same time.

You may also consider replacing entire stem(s). Not overly expensive. Run around $20.00 each in Home Depot. New stem will come with washer, O-rings and packing seal installed. Some kits even include new seats in the package. Diverter stem uses one extra large white washer that is also part of the new stem package.

Hope that helps

Milo