And if this makes any sense to anyone other than me, this is how I would do the 3rd wheel assembly.
Green is a car spindle (rwd car no driveshaft through it)
Blue are ball joints(upper and lower)
red are heim joints
black is frame tube
upper ball joint would be fixed against a load bearing hoop
lower ball joint would be on a single A arm, adjustable for caster
the front heim would be adjustable for toe
The 3rd wheel would be based off of a mcphearson strut assembly, without the actual moving suspension (3rd wheel suspension has a habit of flipping the car mid corner). The structural hoop would replace the strut and be fixed. This design gives maximum adjustability while requiring the least amount of space from the passenger platform