That looks a really nice set of results. My treatment was HDR, EBRT, ADT and I'm only just coming off HT. My onco said "six months check, no point in stressing people out every three months". I don't think my PSA Test results will be very meaningful when you allow for RT/PSA bounce, coming off ADT . So it will probably take a year before my PSA settles down and three month checks would be all over the place at the moment.
Of course yours is more stable and any increase would be more meaningful as you have no prostate. If having tests less often means less anxiety go for six months, if your going to be anxious all the time because you think your PSA may be creeping up and no one is watching then three months.