Maybe a tick higher than the Ü2s (which are already very low to the ground. I measured them at 2 cm at the point under the balls of the foot from the ground to the highest point of the midsole, but they are lower as the footbed is curving inwards on both sides). Like I said, the are very similar except for the front which is considerably wider in the Ü3s.
The Ü3s were supposed to be the next Ü2s (which are the successor of the Feather 3s), but Adidas brought the Ü2s back to the market due to the Ü3s being for a completely different foot type, and many complaining that they cannot buy a premium lightweight Adidas shoe for narrow feet anymore.
What was that you didn't like about the Feathers? I am surprised to hear that, as the Feathers 2/3 are universally praised shoes and were true hit amongst the fans of the speed oriented tennis shoes ( I consider the Feather 3s the best lightweight shoes ever overall an still have 5 pairs of them. They fit me perfectly and do a terrific job).