How to potty train an adult cat that lives with other cats?

First time using this subreddit, so… hello! Here’s my situation:

My family has three cutie patooties,,, 13-year-old Ragdoll cats (Mila, Lola, Felix), two of which (Lola and Felix) are potty-trained. Mila seems to have lost this habit a while ago which makes my mother very upset and also causes other issues. I’ve done some research and have the general idea of what to do, but what I wonder is if the process is any different when the cat that needs to be potty trained has lived with two others her entire life.

We checked with the vet and we’re 100% sure it’s merely a behavioral issue, and not a health issue. It wasn’t all of a sudden, it was a gradual loss of the habit. All three cats are very healthy, physically and mentally. We have three litter boxes, none of the cats have any health issues, except Mila is deaf and she has a small issue with one of her tear ducts, though she’s had it since she was a little baby and all we need to do is clean her eye crust and it’s fine. The litter boxes get cleaned at LEAST twice a day to encourage Mila to use them, but she is an ungrateful little creature (all jokes!!!).

How can we alter the regular potty-training process for Mila? I am a teenager trying my best so don’t be too harsh on me :’)