As a direct sequel to Mega Man 2, MM3 is legitimately pretty good. They've managed to fine tune the boss design to a pretty great level, with difficult tactics required that expressively use your movement and slide dash. The level design is just as good as the previous, with very interesting locales that give you opportunity to use your other robot powers.

It still falls into a major polish trap with certain sections cutting the framerate so awfully that precision platforming is obnoxious unfortunately. Mods can fix this (and there indeed are ones out there) but overall it's still probably the least polished entry so far. The music is also a step down from the previous game, and the last sets of wily levels are very restrictive.

Overall, it's a good action platformer still worth playing. Definitely recommend, even if I prefer MM2 just a tad more.

The nicest way I can put it is that this game is not for me. This game overall feels unfinished and clunky, even when it comes to the weapons. For example, Top Man's weapon is not only terribly designed, but it also doesn't even function properly half the time, sucking up half of your weapon energy randomly in one hit. The extra levels before the wily stages are obvious padding with terrible recreations of the bosses from mm2. Also, this game can softlock you at the beginning of one of the levels, so I hope you are okay with doing all of the wily stages over again, assuming you remember your most recent password.
2 Trons

The slide really makes the series hit it's stride movement wise, and Rush is a great addition. The platforming has been tightened up from 1 & 2 and it feel MUCH more fair. The robot masters and powers are more balanced than in 2 but they're all also a quite a bit less fun and/or useful than weapons from the previous games (especially top spin, that shit sucks). Fuck though man, the Doc robot stages are super tedious, and the final boss is so lame compared to the dope flying alien from 2.

It's length is impressive in general for an NES game, and while I really enjoy it this does cause it to drag towards the latter half with the re-treading of level concepts. The level design feels a little less inspired, but is still really solid here, especially in keeping up with the fast pacing. The OST is also still jamming, specifically Proto Man's theme.

(Also played via Legacy Collection)
Many people consider this a top-tier classic Mega Man game but not me. This game honestly pisses me off, because the first half of the game is super enjoyable. The Robot Master stages have great level design, themes, etc. But then you reach the Dark Robot Master stages, four stages that are an absolute condemnation. They crank up the difficulty by a lot and also drop the quality of level design by a lot, not only that but eight recreations of Mega Man 2 bosses that are just garbage. You need to figure their weaknesses with Mega Man 3 weapons but these assholes move too fast and do too much damage to the point that is nerve-racking having to try and guess what is weak against them considering they can kill you in a matter of seconds. Also the weaknesses to the Robot Masters make no sense and the weapons are lame. The Wily Stages are fine but they don't help, this game doesn't recover after those four agonizing levels and the final boss is real bad.
This game is really saved by its first half and awesome soundtrack, as well as the addition of Rush and the slide, because honestly this might be my least favorite classic Mega Man because of everything sucky about the second half (I can't say Mega Man & Bass is the worst because I haven't played it, but I heard it's pretty bad).

Edit: Okay I played Mega Man & Bass now and yeah that game blows