Basically, the 3-bath kits combine some steps -- reversal takes place in the colour developer IIRC, and the bleach and fix functions are combined. It makes the number of steps smaller, though the overall time for the process is not significantly shorter, from what I recall.
The combination of multiple steps makes it hard (or impossible) to tweak each particular step by adjustments in pH. In addition, combined bleach-fix solutions ("blix") have problems with stability. You need to make sure that the blix step goes to completion.
The 3-bath kits have worked well for me, but I'm just a hobbyist and I don't run control strips for consistency, etc. In addition, I was always sure to use the entire kit (up to its stated capacity) within a few days of mixing it.