How do you get to Under 3 in Megaman Battle Network 3 Blue Version

its a secert

Well, that it may be. In any case, this answer is based on the premise that people know where the Undernet is. Otherwise, they're in trouble. From Undernet 1, go down the middle ramp, down/left at the 3X3 square, and down the long arrow path. Enter Undernet 2. Go right, up the ramp, and take the second warp point. Go down the ramp and reenter Undernet 1. Wrap around and reenter Undernet 2, and from there it's a straight shot to Undernet 3. But shhhh! It's a "secert"!