I think I have a urinary tract infection, is it okay to wait for my gyn/ob appointment in 3 days?

Yes unless. The symptoms are severe. You can always call their office and get a urine sample and prescription on the phone and use the scheduled visit for f/u. This is much wiser especially if you are not otherwise healthy.
Why wait, get tested. Perhaps you can drop of a sterile urine collection for analysis and culture at your gyn's or primary care's office or directly to lab. This way treatment can start 3 days earlier. Will be even more appealing if you have significant pain or discomfort.