I am by no means an expert, but isn't the general recommendation to have at least 120 degrees of chain wrap on each sprocket? I know that what you did is probably done to reduce weight (eliminating the need for 2 other sprockets), but that one sprocket on the corner looks like it has only about 90 degrees of wrap.
If it's not a problem (maybe someone else can answer that for me), then you can leave it alone. But if you decide you want to change it, you wouldn't necessarily have to add the two more sprockets. You could just add a roller that would press the chain inward, creating more contact with the sprocket teeth.
Other than that, looks pretty cool. I like the placement of the motors... it's space efficient and keeps the CG low to the ground.