If his wheels & axles were prepped by DD4H for graphite, then that decision is done (that is what it sounded like). It also sounds like his car might be all built already. speed bump gave him info on COM, my concern now would be alignment, trusting the slots. Without alignment, you could raise one front wheel and have the other drive a rear wheel right into the guide strip and lose every race even though you have great parts.
If the rear wheels ride away from the body (preferably out to the axle head), you can still have a pretty fast 3-wheel car by bending the running front wheel's (DFW) axle to rail ride. Its a pretty simple procedure. If you don't have John's DVD, should still be able to get guidance from the various online forums. League racers by and large drill their rears canted and don't fool with slots. Scouts might have to use them or allow the axles to be inspected.
Most importantly though, much more than running on 3 wheels or rail riding, you have to make sure that your rear wheels are properly aligned. This is where time and tools matter. If you don't have tools (Silver Bullet, drill press, etc.) you may have to 'dress the slots' (re-do them), shim or bend axles to make this happen. John's DVD has a real nice demonstration on checking for this. I think that David Meade's (otherwise somewhat outdated) book in the Scout shops probably has more info on correcting it though.
You can use anything to raise the remaining front wheel (as long as your rules allow you to do that). Extending the slot by drilling it up slightly higher, bending it up, cutting on angle with a hack saw (that half of the slot only), dremel or exacto knife a bit - you don't need to raise it much, ~ 1/32" - 1/16". Push it in tight.
