In the original NES version of this game, you could access secrets by plugging in a second controller. For example, if you hold right on the second controller and press A on the first controller, you'll do a super jump! As another example, If you hold right on the second controller when you fall into a pit, you won't die! However, on the Rockman Complete Works, Mega Man: The Wily Wars, and Mega Man Anniversary Collection versions, you can't use them. It seems to me that this was a sort of debug mode that was accidentally left in the final NES version.
Rockman 3 marked the appearance of nearly every standard to the Rockman formula: Rush the dog, Rockman's slide, Blues, and the occasional reappearance of old Wily Robots. Just remember though: that's most of the standards. Others like the Teleporter Room (go through eight teleporters and beat all the Wily Robots again) and Rock Buster appeared in Rockman 2 and Rockman 4, respectively.