The introduction of a 3rd cat has lead to urination outside of the kitty litter.

2 weeks ago my younger brother has moved in and brought a female cat that is roughly 2 years old with him. My older brother that I also live with has 2 cats. One is a female who is 9 years old the other is a male who is also roughly 2 years old. They can all co exist in close proximity of each other but none of them are close buddies or have formed a strong bond. There's no fighting but if the younger male cat tries to play with the older cat she hisses/growls and runs away but that's about it. The house is big enough for them all to go to their own space and relax when need be. Prior to my younger brother introducing the third cat, there has never been an issue with urination outside of the litter box. We also change the kitty litter daily. And we have 2 litter boxes as well, which were in different rooms up until a couple of days ago but now they both reside in the laundry within close proximity of each other. I noticed a strong smell today when I was cleaning and found 3, there may be more, urination stains on the carpet. 1 was really big and was outside my older brothers bedroom door. The other 2 were smaller and in the lounge room. My younger brothers cat is kinda skittish and avoids human contact for the most part, except for with the younger brother which is to be expected. You can pat her if you approach her but that's about It.

All cats are spayed/desexed.

Is the newer cat trying to mark her territory because she feels uneasy? Or is the older cat trying to mark her territory because she feels threatened? The older cat my brother has had since a kitten and his other male cat he adopted about a year ago. But prior to the introduction of the 3rd cat there has been no issues with urination outside of the litter box.

Any advice or tips would be greatly appreciated.

Edit: changed kitty litter to litter box in some instances because I mixed them up and wanted to clarify.