seller’s agent 6%, and that they only have to share it if there’s a buyer’s agent (i.e., they could keep it all if there is no buyer’s agent).

Maybe that’s not right. But if that’s true, this situation wouldn’t be much different. In either case you just make an offer reflecting a 3% discount, and communicate very clearly to the agent AND THE SELLER that you expect the agent to kick in their second 3% commission to make it work since you found the house without an agent.