Welcome to the boards Major.
I would suggest adding floorstanding speakers as the mains and then using the M3s as surrounds. The fuller overall sound of a floorstander is much better, IMO, than any bookshelf even with a sub. I would recommend the M60 or M80s. You will also most likely find the detail from the M60/M80 easier to hear than from the M3's, at least it was for me.
My next upgrade selection would be to then go with adding a sub as the next best upgrade and then simply adding surrounds as the last upgrade to make.
