I was testing this scenario yesterday. I purchased a pair of fork mounts and attached it to a 3/4" plywood with carriage bolts. There is about 31" of height clearance at the hatch and at the seat (folded down - cargo area slants upward). I have to remove the seat & seat post on my mountain bike. With my wife's hybrid bike, I have to remove the seat & seat post and front handlebar with stem. I can fit two bikes across. It will be tight trying to fit a third bike by staggering (pushed back further between the first two)?
Have to purchase some hardware to finish up the fork mount set up and lashing straps to keep the bikes from moving.