First to take all doubt out the discussion, motorized tricycles, three-wheel motorcycles or Trike are fully legal in Thailand.
The problem with the legality is that most people who want to have a three-wheel motorcycle think of converting a regular two-wheel motorcycle to a three-wheel motorcycle.
But due to the way how Thailand registers vehicles, changing a 2-wheel motorcycle to a 3-wheel motorcycle is like changing a car into a airplane...
Therefore the best way to go, if you want to buy a 3-wheel motorcycle is buying it new or secondhand (from a person that bought it new), which means that the greenbook (Thai vehicle registration) specifies 3-wheels.
A lot of people when you talk about a Trike will think about it as a off-road ATV vehicle with 3-wheels and quickly come to the conclusion that they are illegal for public road use. People like this are partly correct, like a pure (2-wheeled) motocross motorcycle is also illegal for use on the public road, so are off-road Trike ATVs also illegal.
In Thailand a few companies are able to convert new (unregistered) motorcycles to trike's / 3-wheel motorcycles or scooters. You can get the up-to-date information of who're this companies from your local department of land transport.
hi all, there is a reputable falang sidecar and trike manufacturer in pattaya who actually knows what hes doing, he has website ozimoto.com or you can reach him on 0900821232
I see from time to time requests on other expat forums for resource info on where to get a motor-tricycle for a disabled person. They are very common an can be see commonly but where does one buy one.
Maybe a sticky on motorcycle.in.th?