Well if you got a 3" trackball, get a 3" hole saw....
Is that true? For a 3" trackball you want a 3" hole? Wouldn't that allow you to actually lift the trackball straight up out of the CP? It seems like the hole would be smaller than the diameter of the trackball.
Yes, mostly.

If you look at the picture above by Nannuu, you'll see some of the part of the black TB case called the "lip" aka "rim" around the ball. The arcade standard for the outer diameter of the lip is 3" (or metric equiv). The standard inner diameter is ~ 0.25 inches smaller (thus no balls popping out).
Now, if you are going to leave the lip completely under the plexi, then you can cut the plexi hole ~ 2.75 inches. But then the ball is lower (yuck, IMO).
Or if you have happs "High Ball", both the
outer and inner diameters of the lip are bigger, and you'd need to adjust the plexi hole accordingly. But, again IMO, the high ball model pops off the rollers easier than the arcade standards.
All which leads to
this illustration previously ref'ed above.