About to instal a dimmer switch. The old light is a single switch with a ceiling fan (no separate controls for fan). The box has 3 black wires: One going to one screw and two joined together going to the other screw. There are three white wires all connected together with a wire connector.

The new switch has a green grounding wire (and the two screws). I am assuming that I would attach the black wires to the screws, but then where does the green grounding wire attach?

Thanks!

If this is for a fan you cannot install a dimmer. You cannot/should not control a fan motor with a light dimmer. Besides, if you dim it both the light and fan will go up and down.

Green is ground. It has nothing to do with the other wires in the box.
Is there a ground wire in the box? Is it a metal box?

I will instal a separate control for the fan.
The main problem is that I cannot find the grounding wire in the box. The new switch has a green wire... but I don't know where to attach it in the box. There is not a wire that is bare or green... only the 3 blacks that were connected to the old switch and the white neutrals.
Can see a slight bit of copper wire behind two screws in the back of the box, so I assume that they are grounding? Cannot possibly attach this new green wire to them though, they are not accessible.

Get a grounding clip (shown below), use it to attach ground wire to edge of box.