A bush hog is a mower that attaches to the back of a tractor. Technically, the device is a "rotary cutter". Bush Hog is the most well-known manufacturer of rotary cutters, so the term bush hog gets applied to rotary cutters in general, much like we sometimes call photocopiers Xerox machines.
That is out of the way, so your job would consist of driving a tractor with a rotary cutter attached to mow whatever is on this 3 acres. This type of equipment will handle anything from grass up to small trees, depending upon the size of the equipment (cutter and tractor).
Your price will depend upon several factors -- what are you cutting, what kind of equipment, and the condition of the ground.
Cutting grass is much easier and faster than cutting brush or even heavy weeds.
Larger tractor, larger cutter = less time. You might also consider the weather and if the tractor has a cab with AC.
Flat, smooth ground is much easier to deal with than rough, hilly ground. Also, there is a certain danger to cutting on hillsides and land that may have stumps, fence posts or other protrusions. There was a guy killed here a couple of weeks ago when his mower caught on a small stump and flipped the tractor over on him.
So, you need some more information. Ask him how long it took to mow the last time, add a bit as you will go slower your first time and multiply this by what your time is worth. If it has not been mowed before, I would suggest you agree upon an hourly rate and go from there.