I agree the Reliant Robin would likely have been more stable with two front wheels, but I'd have put two together essentially in the same format as it is now. You'd have four wheels, but still the triangular form. I don't know if you'd lose the advantages of the three-point set up, but you might gain some stability.
The Can-Am Spyder motorcycle has that reverse triangle form - two widely spaced wheels in front and one in the rear. Seems extremely stable and for some reason here in Ohio I don't believe you require a three-wheel motorcycle endorsement, just the conventional motorcycle endorsement.
I used to see the Reliant's in the UK they always seemed like a bit of an oddity more than a practical solution. But then I didn't have an opportunity to drive one, so they could have been really cool and a blast in the city.