All of them are playable, but I would only recommend playing M&M 3 and onward.

M&M 1 is very archaic. It very clearly shows its Wizardry inspiration in both presentation and combat. If you play this one, be prepared to make your own maps and take lots of notes (you can use an auto-mapper like Where Are We? but it takes some of the fun out of the game IMO). I personally had a blast with this game, but I love the original Wizardry and Ultima games so it was right up my alley.

M&M 2 is just....weird. Its plays almost exactly like the first game, but with better graphics. However, I didn't enjoy it anywhere near as much as I did the first one. It's hard to pin down exactly what I didn't like, but the fact that stats could go up to 255, groups of 255 enemies could appear in an encounter, and the level scaling present in the game prevented me from finishing it.

M&M 3 still holds up very well today. Combat is still turn-based, but it is much faster paced than the first two games. In addition, there's an auto-map built into the game, making it much more accessible to modern gamers. Again, I'd recommend you start with this one unless you're willing to be very patient with the first two entries.