NOTE: Apparently there is a bit of overscan on the left side of the screen at all times that would have been blocked by CRTs at the time, so there's going to be a noticeable black bar on the left side of all the screenshots. I did some research and played my copy of Legacy Collection and confirmed it's not just a glitch of my emulator. 

Anyway, This one seems to have less changes than Megaman 2. The game has no intro, just a title screen for Rockman 3 and a Copyright Screen for Megaman 3. 

The Robot Master Stages have the same english translation, only sporting the name of the game on the backgrounds of the select screens. After you beat the 8 robots, You have to face the "Doc Robot" which mimics the Megaman 2 bosses. No real text about this, It was probably in the manual.

I grabbed the Interim cutscene that happens after you fight "Break Man" who is really Protoman in disguise. (It was his debut!) Wily runs off with the giant peacekeeping robot Gamma. Uh oh.

In Wily Stage 1, I found a room that was different in the English version vs the Japanese version, and I grabbed a screenshot of each.

As for the ending, it has a different translation, so i grabbed it all, including credits.

For Passwords, I named them based on which bosses I defeated. Hopefully it makes sense.

I've left more savestates for the Wily Levels, as the Passwords dont carry you past the first Wily Level.

Let me know if you need me to do anything with that black bar. Best I could do is cut it off manually.