Hi Nicola

Have you tried changing one of the trays to a completely different kind of litter - i.e. if you have clumping - try one tray with a non clumping or recycled newspaper or wood pellets. Or if your trays are uncovered - try a covered tray and put one of them perhaps in a different place to the other two.

May also be worth getting a plug in Feliway diffuser.

Are your boys neutered?

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Kaz

For kittens, joy is for the asking and smiles are for the giving