The issue really isn't with the VRAM at this point, it's that there's literally no GPU solution that will achieve those results. GPUs just don't have enough power at this point in time to fully utilize a 1440p 120Hz 3 monitor setup.
With 3 1080p 120Hz monitors, it's more than likely possible. The pixel count with 3 1440p monitors is nearly doubly that of three 1080p monitors.