Trimalleolar ankle fracture, just got cast off after six weeks, have no walking cast or boot. Dr said see him in three weeks, when can I start pt?

Soon. I would get the okay from your ortho to be sure, but i would normally start pt as soon as the cast came off and the fracture proved itself stable. The earlier you are able to start, the better.
Your surgeon. If the surgeon took you out of your cast I will assume you have healed. Ask the surgeon for your PT referral.
You would have. To ask your surgeon, he needs to give you the referral anyway.