RE: Best tachometer (that is available)
I am not sure that I understand what is wrong.
If You measure the same propeller with the tach set on two blade and it shows 36, it should show 24 when set on three blade mode.
36 (propeller rpm) x 2 (blades) = 72 (single blade frequency)
24 (propeller rpm) x 3 (blades) = 72 (single blade frequency)
Milos