Hi
You may find it easier of bunnies are neutered. it does take time though.
Maybe start with a couple of litter boxes filled with hay, in the places they usually 'go' to as bunnies like to pee and poo whilst eating:shock:... If you change their litter tray, maybe used a small bit of soiled bedding in the clean tray to encourage the, to use it again..
It will take time as I said and you may still find poos outside but it will get easier... I started with 4 litter trays, now have 2, 1 that they us for nomming , the other they both use to pee and poo in!
Good luck
Fee x