Mega Man 3 has some glitches, too. Let's check them out!
Once you have ShadowBlade, go to it on the weapon select screen. Then just move horizontally, and you have Rush Jet! Keep in mind, he doesn't come with any fuel, as weapon energy is only refilled when you finish a stage.
This is just like the first glitch. This time, do the same thing with SparkShock. You still don't start with any energy, but this doesn't really matter since Rush Marine is scarcely used