That's the water pump pulley.
The secret lies behind the pulley, where the bolts protrude thru slightly.
If you look back there you should see about 2 threads of each bolt coming thru the waterpump shaft flange. Take a large screwdriver and wedge it in the space between the water pump and the flange, and jam it against the end of one of the protruding bolts so the pulley can't turn. Then loosen one of the other two bolts. I've done this many times, works great.

DG