How do you wire a 3 way dimmer switch that has two black wires and one green?

You need to first determine where wires go. If you google 3 Way Switch you can get a diagram. Each switch has 3 contacts.. Think about a kids teeter totter. Each end represents a switch contact and the center is a common point. You connect the ends to either end on the 2nd switch. The common contact on one switch goes to supply power hot side and on the other switch the common point goes to the load. Then the neutral of supply connects to the load neutral and the supply ground connects to the ground of the load.