Driving a 3-wheeled motorbike on car licence

I believe it's to do with how far apart the wheels on the same axle are as to whether it falls within a "car (subcategory)" or a "motorcycle" for driving licence purposes. Certainly for whether or not you need to wear a helmet, the magic distance is 460mm between the areas of contact on the road road (on the same axle). Less than that it only counts as one wheel, more and it counts as two. That doesn't necessarily mean the same rules apply for licencing, taxing, congestion charge etc - because that would be too logical and the bureacrats always try to catch us out.

Motorbike licence holders from before October 2000 should have subcategory B1 (which allows you to ride 3-wheelers, eg a reliant robin, without a car licence - remember the Top Gear episode), regardless of whether they also have a full car licence or not, so should be covered.

If you just have a car licence then you need to make sure it is in fact a 3-wheeler and not a bike. I don't think all models do fall within B1.