I have three 220volt pumps each on there own 20 amp breakers. I want to install an on/off switch for each one in a box on the wall. They each have 2black and a green wire. Can I put all the green wires on one bus bar? I wanted to use a bus bar for ease of making connections.

Yes but you will have to bond the ground buss you install to the existing neutral/gound buss. I would bring all three cables into the panel, run the hots to breaker. Cut two of the ground at some where convenient place and leave the third long enough to reach the existing neutral/ground buss. Strip back the insulation of the third ground to the point of the other grounds. Twist the three grounds together and put a crimp sleeve on them. Then run the longer ground to the existing neutral/ground buss.

Or just cut all three grounds off at the same point, twist together and add a pig tail, put a wire nut on and take pig tail to neutral/ground.

Why not just use the breakers?

Breakers are to far away. I need to shut the pumps off quite frequently to do maintenance.