Many cats prefer one litter box to poop and one to pee.
Cats also do not like to use dirty boxes. Some cats don't like to "share" meaning, if there is another cat's pee or poop in the box, they won't use it.
This can lead to inappropriate eliminations or health issues (holding pee can cause UTIs and kidney problems, holding poop can cause constipation and megacolon)
The "rule of thumb" is one box per cat plus one. One cat, two boxes, two cats three boxes and so on. Unless you want to spend every minute of the day scooping litter, one box for three cats is not going to be enough.
I am assuming there is some specific reason you are taking the kitten so young, so I won't lecture on that (
) until I know why you are doing this. Kittens that young can have a lot of socialization and behavior problems. Luckily you have other cats, so perhaps she won't have too much trouble learning how to be a proper cat.
I live in a tiny one bedroom apartment. I have four cats and five litter boxes, all kept in the same corner, I made a platform from plastic shelving, so they are on two levels. Three on top and two underneath.