The 3 you mention are pretty much the Ford, Chevy and Ram of the RV world. Everyone has a favorite if it's been good to them. Everyone will trash a brand if they had a bad experience. Realistically, they are equal and factors such as ideal floor plan, weight, and local dealer are more important than a few internet complainers or brand fan boys.
Realize these are all basic entry level RVs. They aren't the quality of your house and won't hold up to you living in them full time. There is lots of particle board, cheap materials and generally low grade construction. Oh yeah, they also depreciate like nothing you've ever owned.
All that being said, I love my Keystone and loved the Forest River I had before. For the money you spend the value is great as long as your expectations are grounded. 20-30 nights a year and these RVs should treat you well. Of course, there are folks that get the short end of the statistical curve and have problems but that happens in all areas of life. Do your research on price and be a smart consumer and RVing is a lot of bang for the buck compared to other ways you can spend your "fun money."