How is OpenGL support (specifically OpenGL 3.x) in the different video card drivers available for Linux? Assuming that the hardware supported it well, would the drivers be an issue?
Drivers are definitely an issue. They must basically provide a library that implements the OpenGL entry points via hardware acceleration. Without those, you are stuck with software implementations, or open source drivers (which are not very widespread or successful at the moment).
At the time of writing:
I'm not sure about the other vendors but I would be surprised to learn about OpenGL 3 support on those.