Did they at least increase the exp value of the monsters? If they didn't, I don't see it helping much because you just have to put more monsters in the encounters to spend the exp budget. More monsters that do more damage means PC spending more time on defensive actions or unconcious which will undo the benefit of using lower level monsters.
Here is the logic (and one supported strongly by my own experience with a highly optimized group):
Previously, to present an appropriate challenge for the PCs, I had to simply use higher level encounters, involving either higher level monsters, or larger quantities of lower level enemies.
This meant that for average encounters, it might be a Level +2 fight, and for a boss fight, it might be a Level +4 or +5 fight.
Using more monsters or higher level foes succeeded in presenting more of a threat, but also meant more hps and possibly higher defenses. Hence: the potential for grind. It
also meant that PCs tended to level faster than some might find comfortable - my group was levelling every other session, at this pace.
The common workaround I used was to focus heavily on fragile but dangerous enemies like Artillery. With decent success.
MM3 takes that approach across the board, by making monsters appropriate dangerous from the start.
This means I can now include an @Level encounter and have it provide the PCs the appropriate danger for a standard fight, or a Level +3 encounter for a boss fight.
Thus, less hps to deal with in these fights, so the same danger for the party with less grind. And, ideally, a more reasonable levelling pace.
You don't need to put more monsters in to spend the xp-budget, because you were having to go over the budget in the first place to threaten the PCs! Now, you can spend the budget properly, and everything falls into place.
(As a qualifier, I'm not going to say these issues were all encompassing, nor that every fight with pre-MM3 enemies was trivial for the PCs. It was simply a general trend mostly obvious from a distance, and one that the MM3 goes a long way to fix, for which I am certainly grateful.)