I would guess they don't make them as they wouldn't be practical.

The sideways forces plunging a narrow bit are enormous, that's why the 4mm only plunges 10mm or so. A 3mm bit would snap the first time it was plunged.

I use 3mm (and smaller) cutters on my cnc all the time, but the motion is a smoothly controlled linear path that gradually deepens.

The aggressive plunge and side to side operation of the Domino push the cutters pretty hard.

And the cutter swinging sideways WHILE plunging doesn't impact?

This sort of sideways force is the reason 99% of cutters break.

Mind you, if you scale the DOC as compared to the 4, 5, and 6mm cutters, a 3mm cutter would probably be designed to cut to 3-5mm at best, not very useful in general.

With care, one can easily use the 4mm cutter on 8mm or 9mm thick plywood. I have done three projects with 5mm thick plywood using the 4mm cutter, one example of which is this: https://www.festoolownersgroup.com/member-projects/sorting-trays-(5mm-ply)-done-with-df500-4mm-tenons/#msg662850

Alternatively, the Veritas Domino Joinery Table can be used with a spacer sheet or shims on the table and a 4mm cutter to work on very thin plywood (down to 5mm stock). The Joinery Table, once properly set up, removes the user technique factor.



That said, if someone made a durable 3mm cutter, I'd be interested in it.