Innapropriate Pee`ing

Any advice please.
I have 3 adult Maine Coons, 2 girls and a boy, together since January. Always very good with using litter trays, indoor cats with access to an outside run. Over the last couple of weeks one of the girls has started peeing everywhere. She has peed on nearly every chair in every room, in my sons bed, in the other cats baskets, even in her own. She is still using the litter trays for "routine" toilet use, but if she can get to a chair when you are not looking she will pee on it, very furtively. If you spot her it seems she knows she is doing wrong and runs away.
HELP!!:stressed:

Any changes happened in your normal routine, life style,work times or pattern,any one else joined the household etc,sounds like a stress call for attention...?

Is squatting around a lot and spending more in her litter tray??

Tabitha did the same kinda thing when she had cystitis.

Oh and have they all been spayed/neutered?

I would think if it was cystitis after two weeks she would be quite poorly by now,also they do just little dribbles & by how Wickerman explains things it sounds like full blown pees,also take it that they are neutered being adults...!
Think it still sounds more like a physcological issue.
Did all three come from the same home ?
Could one of them be feeling their feet a bit more & perhaps trying to bully her,the list can be endless.
You could always just get her vet checked to be sure but if she is eating/drinking/playing ok round these "I've done it again times" I think it is more a case of putting yourself into her little mind to find out what is upsetting her.
Joys of cat ownership.

Ive had one girl for a few years, the boy and girl arrived in January. The one doing the peeing is one of the new ones. She was great for the first few months, this has only recently started. All 3 are around 5 years old, all neutered. There doesnt seem to be any tension amongst them, if anything they all get on suprisingly well. It seems she is as good as gold for days, then she has a couple of days peeing on everything she can, then she is fine again??

most vets will tell you that you should have a litter box for each cat ...they might be fine sharing when they are little but they really like there own space. A friend of mine has two catswho shared for 3 years and then the female started peeing on the floor,getting the extra litter box solved the problem. Hope this helps.