Andre and Lorenzo, TracieB - welcome to Purrs.
It's great to hear that you have three trays for your babies - well done. Many people dont realise who important it can be, so you're off to a really good start.
How long have you had the kittens, and have they been kitten checked? Have they exhibited this behaviour from the outset? How do they interact with you and members of your family?
What can you tell us about where they came from, and what (if anything) they were used to there in terms of litter tray arrangements?
What sort of trays do you have - are they open or closed? Where do you position the trays, and what sort of litter do you have in them? Can you describe the tray they favour the most - where it is, what's in it?
How long are the kittens left alone during the day, and where do they have access to?
Not being nosy, and apologies for all of the questions, it's just all of these things can impact on the behaviour in question, and often the smallest of clues can provide valuable information as to why behaviour is occurring, and help you work out what to do to rectify it.
If you can fill us in on some of the blanks it may help point to what's motivating the behaviour. One thing to be sure of, its that cats and kittens dont carry out behaviour like this from spite or wilful misbehaviour - it's more a cat's way of trying to get us to understand that something with their environment isn't right, or doesnt meet their needs. Their needs can be very very subtle too, so it can be a bit taxing trying to work out what's going on.
Also you need to thoroughly clean any areas where they toilet inappropriately, and to do that you really need to use an enzymatic cleaner. RX 66 is ideal, but very hard to get hold of in the UK. We used to be able to get it from www.livingiseasy.co.uk
but I think that site may now be closed. Anyway you can get similar from Pets at Home or the Bettaware catalogue. Washable items can be put in the washing machine, but you'll need a hot wash, so if linens cant be washed on a hot wash or treated in situ with enzymatic cleaner, the chances are you wont be able to eliminate the smell well enough to prevent peeing from recurring.
We had a problem with our two semi feral kittens peeing on the armchair in our bedroom last year, and in our spare bedroom. They were scared of people - badly scared - but they were trying to mark their new territory to make it feel and smell more familar to them. They also decided the chair was great for peeing on, because me and my OH would leave our clothing on the back of it, so it smelled like us, and by marking it, they were effectively claiming us as "theirs." Given there was another adult cat in the house, it made sense to them. They were trying to stake claims. Fortunately, our other cat is very laid back, and just thought it was a bit weird behaviour but excusable in ones so young, and so far beneath him. (He's a saint
We found putting small dishes of food in the areas utilised for inappropriate weeing after the area had been properly cleaned helped. Cats seldom urinate or defecate where they eat, so we began putting just small dishes of food on the item of furniture in question. It worked over a period of about two weeks, and you have to be consistent. I agree, it's not always convenient having dishes of food on items of furniture, but hopefully it wont be for long. Having said that, it's now morphed into a nightly tradition that when we go to bed, two small dishes of food go with us into the bedroom, and we're often woken at night by the sound of happy crunching when the midnight munchies strike.
I also recommend the use of Pet remedy Plug Ins, Pet remedy spray and zylkene in the kittens' food. All of these things will help them to settle in to their new home better , and the plug ins and spray are easy to use. Zylkene is a non prescription food supplement that is milk based - it shouldnt be such a miracle supplement, yet it is.