Read the review on MMX Collection. I beg to differ on Mega Man X3. Specifically, as my younger brother agrees, it sucks! X3 killed the gameplay style of X and X2, and resorted to all manner of cheapness for its difficulty. Many of the barely animated bosses tended to go from one side of the room to the other as their attacks, and the obnoxiously placed enemies respawned too quickly. And as for its music, it was awful! Not catchy like the previous two games at all.
In short, Mega Man X3 was terrible, not fun like the previous two games, and I think it was probably somewhat of a rush job.
X and X2, however, were excellent games IMO. X was a fantastic start to the series, and X2 took everything it did right and really upgraded it (except the music IMO, which was less catchy).
And I definitely agree that the series went downhill after the end of the 16-bit era. Capcom seemed to forget the simple formula that made the Mega Man games (original and X) so much fun, and really convoluted them. I don't like the Zero series at all - they'd be much more fun if they were easier and simpler, without all those upgrades the requirements to get them. I long for the days of X and X2.