You should take a look at my thread in this area with my 4x8' router pics. Here are some pics on the 4x8 project
I'm doing vgrove bearings. Only thing about a 3 wheeled design I could suggest is you should figure where the stagered 3rd wheel (on the bottom) should be in relation to the weight of Z wanting to twist the gantry... (ie you could not have the 3rd wheel directly under either of the other 2 although to far back or forward could cause one of the upper to jump. in my mind) Course weight of the gantry could keep that from happening anyway..
Check out my pics. Also have some ideas of how to hole the bearings and do the adjustments of them..