I am certainly no expert on the Bradley Center, but typically every venue in North America has had their own policies loosely based on U2's prescribed policies. Most of us GA-er's are operating on the assumption that the 3rd leg will operate in a way that is similar to the first. That would be the lottery for entrance to the ellipse, and first come first serve for the rest of the floor. Because of the more limited number of spots that are "good" (along a rail outside the ellipse) many are getting to the venue even earlier than they did on Elevation. I would expect (given that the weather isn't horrible) that there will be people there the day before waiting. I have resigned myself to the fact that I will just hope to get in the ellipse and if not, enjoy the show from the back. It is not worth (to me) waiting potentially 14+ hours for a spot 2 or 3 people deep from the rail. I would rather go enjoy a nice dinner and have a few drinks and then show up halfway through the opening act and take my chances.
Depending on how close you want to be I would get there as early as you physically can. Keep in mind that you will be mentally and physically drained from a day long wait on a sidewalk with little or no creature comforts other than fellow U2 fans, which in certain cities can be comfort enough.
