AoW, HOMM and Disciples all play differently enough to let you enjoy each one of them.
truth.
I own AoW:SW, HOMM3 and Disciples 2 and like them all a very lot. While the concept is the same, the gameplay is entirely different and all of them are interesting.
I only played the demo of Disciples 1 at the time of its release, don't remember why i didnt bought it, but i loved the concept already, leading me to buy nearly immediately after demoplay Disciples 2.
The terrain changing with you conquering specific place is very nicely done and has a good impact on gameplay, the focus on extremely small parties, and their leveling is a nice change in this genre.
Heroes 3 main weakness is the constant need of ferrying troops, it is rather an annoying gameplay part, but everything else is extremely enjoyable and i like replaying often randomly generated scenarios or some of the billions of available user made ones.
I have played for a long time Master of Magic, it is really a cool game even by today standard in term of content.
AoW:SW is the closest to Master of Magic i ever played, while the magic part is less extensive (though some spells are really awesome, especially those affecting the world visuals) than MoM, the battle tactics are better with the units having more possibilities.