It wouldn't be able to fly on three motors only. Opposite motors essentially stabilize each other, both on a horizontal plane and in terms of rotating the quad. So if one motor dies, it would do a combination of spinning out of control, similar to a helicopter without its tail rotor, and rotate on the axis of the two opposite motors that are still running.

And burn too, but a hexacopter can lose a motor...

It wouldn't be able to fly on three motors only. Opposite motors essentially stabilize each other, both on a horizontal plane and in terms of rotating then quad. So if one motor dies, it would do a combination of spinning out of control, similar to a helicopter without its tail rotor, and rotate on the axis of the two opposite motors that are still running.