Do you have the VR sensor sent directly into the MS3 ECU?
I will assume you have a V3.57 board. In this case, turn R56 and R52 trim pots at least 6 turns CCW. This will reset them to lowest limits and you should see a tach signal provided your jumpers are configured properly.
If you are using the MS3X board you should also look at R32 trim pots. These are all 6 turn pots. Meaning they will have min to max resistance value in 6 turns. They will continue to free turn after that but no changes are made. That is a damage prevention function. These trim pots may or may not "click" at each single revolution.
This will be very helpful for you to set the limits on the trim pot.
https://www.diyautotune.com/support/tech/other/megasquirt-vr/