All it takes is available space. If the vehicle has not got it, it needs to be helped along a bit with either a rack or a trailer. A 6 x 4 trailer can be a little small, depending on the trike/s. I have only just started with my 6 x 4 that I intend to fit up for 3 trikes - two facing forward and one facing rearward. It will consist of removable horizontal steel square tubes fixed across the trailer with moveable brackets in which the wheels will sit. The trikes will be lashed or clamped to the brackets for solid securing.
If the horizontal bars supporting the trikes are mounted across the top of the trailer sides, there will be room underneath for carrying camping and other stuff.