The M10 converts fairly easily to 3in.
The loading/ejection port on the bottom is about 1/16" shy of 3" long. It usually works OK as is, but if needed, that 1/16+ in can be carefully removed at the rear from the inside ft flat of the trigger guard group.
That abutment isn't too deep into the action but it's fairly thick so there is some metal to work with.
At the front, the port is clear right up against the breech face so nothing can or needs to be done there.
The action will feed and eject 3" loaded rounds as it is (2 3/4" oal).
If the 3" shells are truly that,,3 inches in length (fired),,no more,,they extract and eject OK. (A touch less all the better!)
Any longer and the fired cases will start to hang up on ejection.
Bbl is pretty stout and a rechamber to 3" along with the forcing cone alteration most likely doesn't approach slim wall dimensions but careful measurements before any such work should be done first to be sure.
I haven't seen any X-ray vision'd gunsmiths yet though I've seen their work around.
