The picture I was looking at first appeared as though the outside end was on the ground, meaning it needed to be picked up and slid back in. That is clearly not the case! My 3rd bearing is in the middle of the track and keeps it supported when the first set of bearings roll off and out of the track! No need to lift it and roll it back in. It stays the same level all thru the travel!
I see what you're saying. All your 3 sets of bearings are attached to the tray. Your first set of bearings "roll off" the tracks when extended and if there are only two sets, the front of the tray will fall down. This is where you needed the 3rd set. Does the center set of rollers roll off the tracts too when the tray is completely extended out? Are they located closer to the rear bearings?
I designed the roller locations differently wherein they are always in constant "work".
2 pairs are fixed on the frame and 3rd pair is fixed on the back of the tray and slides with it. This 3rd pair (on the tray) holds the back in place when tray is not extended. No bearings fall off the tract at any given time. On full extention, the 3 sets of bearings become adjacent to each other for added strength and leverage.