I never had a problem with GW paints (they are GW?). Reaper had some issues with the first releases of some of the Master Series colors, but they should be sorted out now.
3:1 (3 water to 1 paint?) is not a bad starting point for most acrylic model painting. Someone told me a long time ago that if you can cover in one coat, you need to thin your paints more.
Yellows of any brand are notorious for not covering. It has to do with the pigments involved. Just be prepared to put on lots of layers to get coverage.
Latex paints also have a certain period where they are set on the mini, but very tender. Applying another coat tends to actually tear the layer underneath and cause all sorts of bad things. Put the other layer sooner and you have wet blending, wait a bit longer and you are layering.
Hope I\'ve helped.