[cypher]

I think probably the only way to stop them peeing in the tubes would be to remove them tbh.
I would do that, put some wet substrate in the litter tray on top of some chinchilla sand (not dust) & they should begin to use it if they pee somewhere else move the tray to that spot & put wet substrate in it again.
Having to clean three times a day sounds like a lot of pee & they normally only pee a small amount, how many do you have in there?
If you do remove the tubes you should be able to keep cleaning to a minimum so they won't feel the need to constantly remark their territory.