I assume these are all adult cats. They can take longer to warm up to each other than kittens.
It sounds like you've done a great job introducing them over a period of weeks, so it's probably just a matter of them needing more time to accept each other. When they're all together, offer some new cat toys, treats, things to climb on or whatever it takes to try to get some positive interaction and distractions from hissing, etc. It's likely they'll start to get along better with a little more time.