Perhaps they don't like the smell of it.  That is, one or more of them might
not care for a scented litter.

Just a thought.

Personally, I prefer to use an un-scented litter, change it once a week
(more if needed) and keep it in the bathroom where everyone does the same
thing..  keeps the confusion factor at a minimum.

They also eat and drink in the kitchen, where we all eat and drink.  I think
having these two things in place makes it easier on everyone - we -all- do
the same things, in the same places.  sleeping as well.  I have one hell of
a time sleeping when my fuzzy kids aren't in bed with me.

Of course, sometimes in the middle of the night, there is a bit of quiet
giggling from a few doors down the hall and quite a bit of heavy purring,
too.  But I smile to myself when that happens, and sleep quite well because
of it.

Do you clean the boxes every day?
You should add another box for three cats, one that's large enough!

Carola

We had 3 cats and had 4 large rubbermaid storage boxes.  We had to
scoop them 3 times a day (or if possible, every time they were used)
because 1 cat refused to use the box if there was anything in it, even
if it was his.  So try adding more boxes so he can find one that is
unused, and try scooping their business out as soon as they do it.

Everyone has some good ideas, I am wondering if you know which cat it
is?

My little Nikki (Nicodemus RB) used the box religiously, but when she
finally lost all sight in her eyes, she had trouble "hitting" in the
box.  She'd be in it, but not quite.  Of course, she'd come and get me
to clean up her boo-boo...  The vet wanted her put down (just because
she was blind?  No, but thanks for the suggestion...  New Vet) because
he thought she couldn't have a good quality of life.