well since m980 hopes me to reply then...
MLRPG 3 is way better than MLRPG 2... in some way. For example, touch screen feature is used much more than MLRPG 2. (Know that the only place you use your stylus in MLRPG 2 is in Star Shrine to remove the dirty spots on the paper) For example, Bowser's special attacks all use stylus to operate; Jigsaw puzzle appeared later in the game; A few battles need you to turn your DS and control Big Bowser to destroy a few specific enemies, etc etc. And, Mario and Luigi sometimes appear inside Bowser(usually they are needed to play small games to boost up Bowser's Power or revive him; While they appear outside Bowser it's just regular MLRPG, this is very creative.
Uhh... About some disappointment... There are a few. Like this time some tasks that need you to explore the whole region to collect some pieces are really annoying and time consuming, which I don't think they are fun.
Overall, you shouldn't miss this game and I think it's at least at the standard of MLRPG 1, so it won't diappoint you!