Accepted by potential thesis supervisor while still waiting to hear back from two others. How to respond?
I think it's perfectly ok to let the professor who's accepted you know that you are waiting to hear back from another opportunity before deciding what to do. There's no need to be any more specific than that. Ask the professor if there's a deadline that you need to tell him your decision by. If it's soon, you could even let the other professors know that you have an offer in hand and need to decide soon. This may speed up their decision.

Don't worry that the professors will think negatively towards you for this. It's a very common situation to be in, and academics understand that students will have their own preferences for who they want to work with. Just make sure you are polite and not pushy in your communications with them.