The attaching points should be triangulated that's why they use 3 mounting points. You can get this out if you go about it the right way, a screwdriver probably wont allow enough force (leverage) to be generated. Hang in there.
If the vise grips didn't work USE HEAT and penetrating
oil
A breaker bar will shear it off.
Soak it and walk away for a while.
SOAK IT SOAK IT SOAK IT HEAT HEAT HEAT TAKE YOUR TIME
Rick