I think there's a lot of great advice here already, so don't want to add too much - except to echo a couple of points:
1) give them time - Riley and Skye are only now getting on well 70% of the time (there are still minor disputes, but nothing serious), and Skye came to us 2 months ago.
2) Feliway can help a LOT - not for every cat, but it's worth a go
3) Make a HUGE fuss of the resident cat - extra treats, hugs, toys, bowls, play sessions, litter trays, hidey-holes, the works. She'll be feeling insecure - If she comes to associate the new arrivals with an increase of good things, it should help her relax.
4) Try to set aside a "neutral" room / rooms, where none of the cats are usually allowed, and have them take turns in it to smell each other. have treats and play sessions in this space, so they associate the other's smell with positive things. Try a few short meeting sessions in this space, closely supervised, with LOTS of treats and reassurance from you.
5) try to stay calm, be patient, and keep trying!
Lots and lots of luck!