[Untamed]

Chalk line, black sharpie marker, steel T-square, circular saw, carbide blades (2-3 because they wear down kinda quick), 8 lb hammer and cold steel chisel, bucket of water and sponge.

Measure the hole then chalk them. You'll need a partner to hold down one end of the line. Make sure your lines are square by using the t-square. Go right over those chalk lines using the black marker. If you try to cut along the chalk line, the chalk will blow away from the saw action. Take your time to make the line as straight as possible - and expect to throw away the marker after because of hte chalk dust on it.

Set the depth of the blade then cut along those lines using the circular saw. At 3 inches, should be easy to do. Once the outline is cut, cross cuts will make the hammer and chisel work much easier. This is why you need more than 2 blades because they wear down kinda quick. If you dont know how to do a plunge cut with a circ blade for the cross cuts, then hammer and chisel a section out first, then do your cuts.

Hammer and chisel out the pieces. At 3 inches deep and with cross cuts it shouldnt be too hard. Below the concrete there is probably mechatum or a rock and sand base. That will either be easy but tiring, removing it by hand with a shovel, or it may require a jack hammer. Your call. Measure the depth as you go along to achieve your 12 inch trench.

I dug a trench in my driveway to reroute the sump pump pipes, and built a concrete patio in my backyard. 12,300 pounds of concrete! Concrete is the bane of my existence!

EDIT: Safety glasses!!! And maybe something to cover your nose and mouth. I imagine a garage floor, usually dry, would be very dusty when cut. Save your eyes and lungs.