I experience the same thing. Two figures (and all their clothing) fits nicely whereas three puts it over the card's limit and it drops to CPU processing. While mine doesn't crash, it keeps restarting the render at some point. The image gets better and better, indicating that the render is continuing, I just have to cancel and save it at some point.

I'm not impressed with having to use a proprietary renderer and am watching the gents at Daz3D who are writing a plugin to use the OpenGL(?) of some open source renderer.