Why do Trembuckers have three mounting holes per side?

[Ashurbanipal]

Mm, Ibanez did this back in the 80s especially, so you could get the tilt of the pu just right. Never had to do it, personally, as the angle of the rings tended to be cut fine so the pu was fairly parallel with the strings/pickguard, especially on a trem guitar.

The oem pu legs had three holes on the bass side, but the mounting rings were always only tapped for the two outer ones.