You can do it, but it'll be very load limited.
The FAA can approve the door modification (the door will swing upward, hinged on top), or the door can be removed entirely.
Even with minimal fuel (three inches on the dip meter), however, my guess is that you'll be over gross with three normal size jumpers and you as pilot.
On the other hand, I've jumped out of a normal 172 by just pushing out the door, but it isn't something you'd want to do more than once.