I have 2 young male cats about 1 and a half years old. They haven't been
desexed yet and they will be in a couple of months.

Now there is a slightly older desexed male that lives next door and only
recently has he started to chase my two down and start attacking them. My
two don't even have to go near him or even go into his territory???

Is there anything that I should know about here, apart from my two needing
to be dexed?  They are not new to each other, and have lived next door to
each other since they were born.

Thank-you

Nat