Turntable leveling...

Exactly !
If I was in your position, I would made a second plinth out of granit with 3 adjustable spikes (or cones or whatever) in between your TT & your marble stand.
Always check the level to your armboard also. IT HAS TO BE exactly the same together with your platter's : INEVITABLY QUITE SIMPLY P E R F E C T
This is critical !

Three vs four has been discussed.
Three is great until you bump it. Four is really stable.
For turntables three is OK. For electronics I do not like it as stacking. or heavy stuff high up is really dangerous. As for speakers. well, as long as you never knock them over... three is possible.
For racks it makes no sense for three, as the two hanging corners suck.
I like FIVE personally, and add a rubber fifth foot under most of my stuff to dampen the chassis.

Level the table and Be fanatical about it! Coins, paper etc. all might work. Three or four is an interesting subject. In most cases I prefer three though the other day my deck almost fell off; could've done it with four too.

I will first get the cabinet level and then level the TT. It is a shame that Rega does not allow the feet to be adjustable. I tried spikes on my platform intially but the table would pick up noise from my cabionet when I touched the drawers or doors. I replaced the spike with Vibrapods.