Cats have started to argue

A few nights ago, we woke up in the middle of the night to hear our two female cats fighting. Normally they fight as play, but this sounded serious. There was a lot of hissing and growling at each other. Then last night they were hissing at each other. They've been together for almost 3 years now, and this is the first instance of them fighting. Any thoughts. There have been no changes to the house, other than we've temporarily moved the ferrets downstairs.

what are the sexes of the cats? are they fixed? mating for cats can sound like some dangerous business is going down... i've seen them mate, they don't play around.

The two that are fighting are females, both fixed. We also have a fixed male in the house. Last night I heard some hissing but it wasn't much. These cats have always been peaceful towards each other, so it is very worrying to me to see them fighting or hissing at each other.