Twin bottle for M3

Why wouldn't it? the bottles are not changing and the adapter fits on the action in lieu of the bottle so I guess that it would fit.

No the 2nd reg is in the bottle adaptor. which is removed when fitting the dual bottle adaptor kit you would need one for an M3

and yes the M3 bottle fitting is new and not compatible with Mk2 Ect and vice versa

You would be better off with the 480/500 or 580 single bottle unless you were going for what it looks like over practicality. Or carry the second bottle as a spare. Less to go wrong.