Hello,
In the company I work for, here's what the FOM says:
***RMP 1 is for VHF 1 only; Change VHF 1 frequency only on RMP 1.
***Monitor the emergency frequency 121.50 MHZ on VHF 2 at all times.
***VHF 3 is normally set to ACARS. If necessary, change VHF 3 frequency through RMP 2.