Does your oncologist have imaging scheduled after third cycle? My husband was on a cocktail of Jevtana and carboplatin for all 8 cycles, but one of his oncologists had recommended starting on Jevtana and adding in the carboplatin after 3 cycles if his scans warranted. His other oncologists felt his cancer was too aggressive and needed to add the carboplatin right away.
So, ask about a PSMA scan, and that should indicate your next steps.
I know that PSA initially rises due to killing off of cancer cells, so the scans are the best indicator.