Most craft I know have 2 VHF and maybe a HF. 1 VHF com is needed for general communication in Area/s so I think the other should monitor 121.5 as a HF can not.
If you have 3 VHF that is great...
You don't need 3 VHFs to monitor 3 VHF frequencies.
There have been VHF RX/TX products available for quite some time that have the function of monitoring a different frequency to the active frequency. E.g.:
https://buy.garmin.com/en-US/US/in-t.../prod6440.html. I note this from the link:
The [product's] frequency-monitoring function gives you the ability to monitor ATIS or the 121.5 emergency frequency without leaving your assigned ATC channel. This allows you to listen to standby frequencies while giving priority to the active channel, meaning you'll never miss a transmission.
2 of those products on board and you can listen to 4 VHF frequencies!