To tip the truck, it must be tilted until its CG no longer is above its footprint (support base). The truck on the left is already in trouble, and will tip. The other trucks have CGs above their own support bases and are safe. In this particular situation, the third truck is "most safe". However, if the incline sloped the other way, it would be the first to tip. Typically speaking, a truck with a low, centered CG is the safest across all situations.