Due to the challenging physics in GRIP we have a number of constraints on the wheel configuration. The moment the vehicle becomes even a tiny bit unbalanced a lot of very bad things happen. Symmetry is really important, though we've been exploring ways of having a physics configuration for vehicles that is independent of its visual representation. Still, they have to be fairly similar otherwise the vehicle will just behave implausibly.
What this means though, is that while it's unlikely we'll ever see 3 wheeled vehicles in GRIP, you will start to see vehicles with more than 4 wheels coming soon