A family member has had to move in with me out of the blue. I have two male cats and she has one, we've been keeping them seperated for the past 3 weeks. They can see each other through a window and should be used to each others smells by now. My cats have started hissing at each other - they've never done this before. I think 3 weeks is long enough, how should I introduce them? Two cats at a time or all three?