If you don't use the tractor's hydraulics, there would be more to it than just adding a PTO pump. You would have to add a reservoir of at least a gallon per GPM of pump. Then you should filter the fluid so you add that. Then if you are adding the reservoir you should have it attached to the backhoe so that when you detach it all stays with the hoe and you just need to disconnect the pump at the PTO. Mounting all this means you could compromise the movement of one of your backhoe functions if you don't have room.
As far as attachment style a frame mount is much preferred over a 3 pt in my book. It makes a much more solid connection when you dig and you need that to dig effectively.