They can wind up on their sides Jabber, have you ever watched Mr. Bean? One wheel in front does make a lot of sense in terms of manouevrability however. Scarabs could be adapted as tow-trucks and were particularly useful in narrow confines (such as tunnels) for their ability to turn in almost their own length.
An oft-unrealised problem for tri-wheelers is when driving in heavy snow. Four-wheelers can follow the previous car's tracks and avoid getting stuck; but when your 3rd wheel is in the middle.....! Similar problems arise on unmade tracks.
GG