3 months seems way too soon for doubles tennis. Very surprised your surgeon would suggest that at 3 months.
I had a MUA at 14 weeks. My ROM became very limited due to scar tissue. The MUA loosened my knee up considerably. Started daily PT the night of the MUA along with a "Dyna Splint" for extension. I am currently at the end of the second week after mua and my gait is considerably better as is my ROM.
I too am looking forward to getting back to doubles tennis, however, I don't think it will be anytime soon
Does your surgeon say what could be causing the buckling?