v3 does not change between rms and avg modes...
Its always in rms.
the v2 was the one that you could choose between the 2 modes.
The firmware version of you vamo v3-v5 determines whether you have RMS/Mean mode selection.
To check the firmware version of your v3-v5:
Remove the Battery Cap
Press and Hold the Fire Button
Reinstall the Battery Cap
KSD 2.0 or KSD 3.0 should be displayed on the OLED display
Story is that KSD 3.0 allows switching Mean/RMS (verified on my Vamo V3) while KSD 2.0 does not (RMS only).
This being true explains why some V3s won't switch Mean/RMS.
A PBusardo Review - The VAMO V5 - YouTube
PBtusardo V5 review shows how to switch Mean/RMS