I believe in theory if you mechanically connect the motors and assemble them such that they are identically oriented and all three motors are identical and the ESCs will all behave identically, it would work great. In reality that isn't going to happen. That said it should still work, though I would just expect a lot of inefficiency, thus heat, thus reliability issues.

This, but with 1/5 scale inrunners, is exactly what Jerome Miles did for his heavy bar spinner last Robogames. It had buckets of power but couldn't get past 0rpm without help. A regular go kart centrifugal clutch (30 quid) should solve that. Also this sort of thing wouldn't exactly be cheap anyway, talking 3 motors and ESCs + spares, it's not too different in cost to more traditional approaches. You're also a bit limited in voltage with cheap hobby brushless stuff until you go bigger than NTMs, which also pumps the price.