Well they wouldn't have bothered with RT if they knew it had spread. Chemo, RT and HT all together is throwing everything at it so they know you are very high risk.
If they did get it all the three years HT will be enough and you may live anoth 20-30 years. If when you come off HT your PSA starts to rise it will be HT for life and getting beyond five years is doubtful. So on average you will live 15 years, but in reality you will either live 5 or 25. As you don't know which it will be, you may as well assume 15 years until you get test results to show it has gone one way or the other.