Yes, it is common to wait 3 months for the first uPSA test, even with adverse pathology like yours. This is because it's not unusual for PSA levels to continue to drop over those first 3 months before reaching nadir during the 3-to-6 month period.

That said, @wheel1 makes valid points and I've seen at least one study claiming that the 6-week PSA level has real prognostic value in terms of assigning a probability to eventual recurrence, so you may wish to have that data point in hand.

As far as treatment goes, however, adjuvant radiotherapy is rarely given these days, ever since the RADICALS study rigorously established that there was no long-term survival benefit to adjuvant treatment over early salvage. Waiting 3 months is not only not going to make things worse for you, it will be giving your body essential time to heal from surgery. You want to achieve some baseline recovery and ideally regain full continence before starting radiotherapy.

The fact that all your lymph nodes tested negative and you were staged as T2 is very good news, even with the cribriform and positive margin.

Best of luck.