When I'm trying to encourage new ferrets to use the litter tray, I pick up (using a plastic bag!) the poo they've done out of the tray and drop it in the tray, then clean where they had originally done it! This way when they walk in the litter tray they'll smell their poo and hopefully think "Oh, that's my poo......this must be where I do my toilet business"........well that's the theory anyway!
My girls use just 1 litter tray in their indoor cage, they're very good and go to the bottom level of their cage every time to do their business! However, when 3 of them were in the outdoor hutch