The SBE I uses the Beretta/Benelli Mobil Choke system with the threading at the top of barrel.
Yes this and any Beretta/Benelli Mobil choke will fit the A300
Yes this and any Beretta/Benelli Mobil choke will fir the M3000.
The Briley Choke Tubes Interchange Guide says
Tri Star (all gauges) - Benelli Crio Plus, Benelli/Beretta Mobil
Are compatible.
Yes, these are Mobil pattern chokes.
The Benelli M2 shotgun uses the "Crio" choke tubes
https://www.opticsplanet.com/carlsons-choke-tubes-12-gauge-benelli-crio-plus-ported-choke-for-sporting-clays.html
While a slug can be shot out of most choke tubes, the tighter the choke the greater the chance of an issue. I would definitely not recommend shooting a slug through a full or extra full choke. Most people do not use a slug with chokes tighter than an Improved Cylinder. Consult your owner's manual or contact the firearm manufacturer for the best info.
No, it would not fit. We recommend this product instead: https://www.opticsplanet.com/carlson-s-choke-tubes-cremator-waterfowl-beretta-benelli-choke-tubes.html