We don't have an M3, so probably won't be doing any testing. We can't say for sure whether it will work on the M3, but the first person who tries it will tell us whether it works.
The stocks are the same for the M1 and M3, both Benelli and Speedfeed stocks for the M1 work with the M3. The Speedfeed stock for the Benelli is basically an 870 stock with a adapter plate between it and the receiver, it doesn't look very clean but until now it was the only shorter LOP stock available. I'm looking forward to trying the Urbino stock on my M3.