I don't think I would think too fondly of a so called 3 horse trailer with 3500lb axles. Is that even legal ? Think about it, 3500 lbs of trailer, plus 3 horses, plus tack ? Maybe some water ? Your overloading. I thought all 3 horse trailers had to come with minumum 5000lb axles.
Vans come in all shapes and sizes. You can get some pretty heavy duty rigs. The van at work is a 3/4 ton with a diesel, pretty sure they are rated like trucks, IE, F150 F250,,,, ford vans are E150 E250 E350, I imagine they have similar tow ratings. I think if I could go back in time I woulda bought a E250 and kept my bumper pull, and fixed up the van as a sleeper. One driver could stretch out and get real sleep on the road.