Please get both your older cats to the vet and have their kidneys checked. Peeing outside the box can be a sign of kidney disease and once it progresses, it can be difficult to recover (I lost a 6 year old dog to kidney disease a year ago).
If that isn't it, you'll have to experiment. I would suggest putting one cat at a time in a bathroom (or a dog crate if you have one) with their litter box and see if you still have the problem. If so, try this approach with the next cat. I find vinegar and water works well. But, you may have to throw out the rugs, as those can be tough to get clean AND the cat will smell the urine and continue to pee on the rug. Maybe try some cheap towels (Costco has like 8 for $17 bucks - I use those for my boys to take to school for showering after sports since they're cheap and colored. The towels will come clean in the wash easier than a rug, especially in hot water.
It can take a while for cats to adjust to a kitten - we found a kitten over a year ago and our other cats are finally accepting her, BUT our kitten can be a monster and she does harass the older cats that don't want to be bothered. All my cats use the same litter box with no problem, so I don't think another litter box will solve your problem.