I could really use some advice

Hi all. I'm new, and I'd be really appreciative of some advice. I have had two cats, ages 8 and 9, for going on 5 years now. We recently decided to expand our feline family (last one) and adopted a 4 month old female. We kept the kitten seperate from the other two for a week, scent swapped, etc before introducing them. They've done okay. My 8 year old couldn't care less. My 9 year old will hiss occasionally when kitten steps over boundaries, but no fights.

The problem I'm encountering is litter. I have four boxes total. Kitten has completel stopped using hers, and just wants to go in our resident cats' boxes. The issue with that is now my 8 and 9 year old are unsure about using their boxes. There have been no accidents, but I've been watching them and they'll step into the box, scratch around, smell, then hop out. There was one box that has been kitten free that they both went into (refused the other 3 boxes) but recently kitten eliminated in that one, and they haven't pottied since.

I've kept the boxes really clean. I scoop it each time kitten goes. I'm just trying to figure out a solution before we have a problem with them going outside the box. Is it an issue they'll eventually work out? None of them are going in two of the boxes. Kitten wants to go in the 2 boxes resident cats have been going in since we've had them. Any suggestions? Thanks!

Welcome to TCS!  Congratulations on your new addition, even though it's causing some stress in your home.  What you're experiencing is typical behavior as they are working add the addition to the clan.  I would suggest adding at least one additional box to your existing boxes.  Ideally, you should have one more box than you have cats. I know they're not using two of them, but to be on the safe side, add one more.  Also, you might try adding some Feliway diffusers to your home.  That will help everyone with the stress and can really aid in litter box/elimination issues.