Introducing stray to household with two cats

Question
Have an older male cat and young female - no problems with these at all.  Recently stray appeared with no encouragement it stayed.  It moved into one of the outside sheds.  Obviously in this weather we couldn't ignore it and have made it as comfortable as possible.  Its fed regularly.  As its decided to stay we would actually like to get it indoors where its warm but it does not seem to get on with the male in particular - the continually hiss at each other.  Haven't clue about pregnant cats but this cats stomach appears very taught and solid without being fat.  A friend suggested that she may be pregnant.  Its a lovely cat and has been around this area for past 4 months.
We are more than happy to take it on but need help with integrating with other two cats


Answer
First I recommend having her tested for Feline leukemia and FIV.  This can be accomplished by a simple blood test your veterinarian can run within a few minutes.  She may outwardly appear healthy but may carry the virus, and you don't want to take the chance of infecting your other cats.  Cats can often be very reluctant to accept to members into the household.  Sometimes it can take days for them to accept each other.  As to whether or not she's pregnant that's hard to say without seeing her.  If her nipples are swollen and she appears to be lactating then she is pregnant and will be having them within the week.  But there are a couple of other things that can cause the taut belly.  Sometimes they can have an enlarged liver that can cause their abdomen to swell and sometimes its just because they're overweight.  Your vet should be able to tell you more definitively.  Once you are ready to introduce her, I would do it slowly.  Bring her in in a carrier so that your other cats can see and smell her, but not hurt her.  If they seem to be handling that ok then you can let her out, but I would keep them under close supervision for the first 24 hours at least.  Its pretty normal for cats to hiss at each other when they don't know each other, so as long as they aren't actually fighting I wouldn't be worried about that.  With adult cats sometimes it can take weeks before they finally get used to each other, or at least used to the idea that they are stuck with each other.

Amanda