I have several of the Atlas switchers. They are very affordable, but then I run them on 2 rail. The only real change for is putting a scale coupler on it. I do like the slightly larger Atlas ones quite a bit as well.
For conversion to 3 rail, it wouldn't be impossible, but for the effort Gunrunnerjohn makes a good point about the K-line switchers. They seems to be selling at a premium these days, but nice little critters.