No front number plate for three wheelers? What all three wheelers? Are you sure you dont mean just 50cc moped registered ones?
Its all to do with the weight, the same as the fitting of seatbelts rule. If a three-wheeler is under (I think) 450kg, regardless of age, it does not need either seatbelts or front number plate, so stuff like Messerschmitt KR200's do not need a front plate, but if the vehicle is over that weight, ie, Reliant Jubilee Robin, it DOES require a front number plate.
Its all to do with the old head-in-the-sand rulemakers beliving no motorcycle can weigh more than 450kg & no car can weigh less than 450kg, hence all vehicles UNDER 450kg must therefore be of motorcycle origins & not need a front plate & all vehicles OVER 450kg must be of car origins & therefore has to have that plate.