The three you are already working with are pretty good - though Mainstreet have been a bit hit and miss for me, as have most of the others I've worked with.
I've been lucky with Revenue Network (formally Betting Partners), though they are not the best to work with.
But I'm only a tiny affiliate in the grand scheme of things - hopefully other affs will have some better input.
If you're looking for reputable US casinos that pay both players and affiliates, give me a shout. I run LegendAffiliates.com and currently we have 4 US friendly brands.