Try a feliway diffuser. Its what my vet recommended, its worked to an extent for my neighbours sibling sisters but not a bit of difference for mine.

Give it time, try not to intervene in any spats...they have to establish a pecking order, if they can do that peace should reign.

No one can say how it will turn out. Cat sanctuaries have dozens of cats live in harmony and my mere 5 cannot stand each other. At best there is an uneasy truce but Jasper is constantly snarling at Tabs, outside, Tabs picks on Floss and Oscar but wont in the house. Floss is top boss inside but Jasper occasionally tries to oust her. Even my meek and mild Fliss snarls at Tabs (not a popular boy with any cat) I think my cats problems are largely down to Tabs being a dirty fighter, wont have a head on confrontation, prefers to jump them from behind and cats have long memories...if only they would have a straight out spat we might have peace.

I had to help out my sister by taking her cat in for a few months. We did the introduction by confining her cat to a room with his own litterbox, toys, and food/water dishes for a week. We'd go in and play with him and hang out so he didn't get lonely. When he felt comfortable, we encouraged a little cracked-open-door sniffing. There was a lot of hissing on either side of the door while our 2 cats got used to the idea of an invader. Then we left the door open one day and stood back to let them figure it out. For a while, that room was his 'safe' room, where he would retreat if he wanted private space. The other 2 cats let him alone in there. They all got to be buds, but the hissing and strutting did take a few weeks to calm down completely. I think the separate room and slow introduction made a big difference. Maybe it's not too late to do it for your cats?