3Gb is a lot but then so is 3 monitors.
It's going to depend almost entirely on the games, with such a high resolution 11 MILLION pixels the VRAM usage will be through the roof especially if Anti-Aliasing is used, the thing you have to factor in though is that to get acceptable performance with 11 Million pixels you need to throw a lot of GPU power at it.
This idea of running out of VRAM is one to keep in mind, but with modern games I suspect the GPUs are going to be struggling to keep performance playable before you actually run out of VRAM.
In short I suspect there's very some games that will be able to max out the VRAM, I think those at risk of doing so are more likely to be running into FPS issues long before VRAM is a concern, for example you might not be able to use 8xAA due to VRAM limitations but if your GPUs can only provide like 10fps in this config anyway then who cares.
You can't share monitors/VRAM like that,