I find MM3 is a bit more difficult to get started with; the combats seem harder in the first areas. If I were you, I'd go for MM4 (Play World of Xeen and don't teleport at any pyramids) which is easier to get started in, and is short and more-or-less self-contained. If you like it you can decide whether you want to go on and do MM5 with your pre-levelled MM4 characters (which helps a great deal), or go back to MM3 to fill yourself in on the back-story that MM5 builds on.

But I guess it depends what you like. If you find that you can't cope with the old 2D engine, MM6 is a very good jumping-on point.

I seem to be one of the three fanboys of the 3DO might and magic games out there (including Might and Magic VI: The mandate of Heaven, included in the GOG package). Still, these have the most modern interface, and if the games you are into generally have a fully 3d world, then this is your best bet as far as the Might and Mgaic games in general, or at least that are available on GOG. If you don't have a dedication to any particular Might and Magic game, this is where you should start, and thereafter, you should go backwards. There are several good reasons, not least of which MMVI is the only one of the gog games that is fully 3D.

But XEEN is a very enjoyable game, and I'd say it was the second easiest for a 3d-world gamer to get into; although it is turn-based and functions on a grid system, it is still highly graphical. The graphics in Isles of Terra are less advanced, but as far as I can tell it is similar to XEEN

In these later games, you can see monsters as you approach. World of XEEN even has an automap that you can check to see where you are, and decide where you want to go. The first two games have to be mapped by hand and can involve unwinnable battles even early on, so be careful with those. (One of the first three games also lets you map automaticlly IF and only if you have a certain skill, but I forget which one).

The old games DO have their own beauty; they are often rated as a blend of strategy/rpg, which fits my experience as a 3DO philistine. Naturally, as they were created more than 20 years ago, the graphics are less than stellar by today's standards, as well as being sold to a different gaming community. But they contain important bits of continuity, so after becoming famliar with the series and the earlier interface, I would cartainly not forget to tackle those as well. The fact that GOG has ported these old classics to NTFS operating systems is no small boon, although I would not call them beginner level. They are very challenging.

Working your way backwards is the easiest way to go, although doing them in the "correct" order is a "new" challenge for a 3d-world gamer like myself.

Finally, however, if time investment is not up your alley, I'd say go for a game set in a truly 3d world. Even if it has important plot developments, there are often ways around those and you can just murder a bunch of innocent peasants (you sick bastard) no matter where you are in MMVI. While it really is not JUST about killing stuff, MMVI is the closest thing in a Might and Magic game where you can go if that's all you feel like doing. More graphic 3DO games, however, are probably what you're looking for if that's all you care about.