Yes, it is enough. The resolution by itself doesn't increase memory usage quite as much as most people believe, but some effects and filter are memory intensive and may cause the frame buffer to escalate quickly in size.
The thing is, higher resolutions produce a finer image on their own, decreasing the need for further filtering. While one may want to use 4x multi-sampling antialiasing in 1080p resolution, it might not make that much of a difference (as opposed to 2x) at 1440p. Still, 3GB VRam should be enough for standard game settings (I mean up to the best in-game setting, without touching extra IQ features from drivers).
Naturally, some games will create a perfect storm in which you might hit a wall (such as Metro LL with uber settings), but that usually happens because there are lots of unneeded filters and effects overlapping (in the Metro LL case, for instance, supersampling on top of all the image quality settings is known to produce next-to-zero image gain over 4xmsaa, while dragging down your hardware to a halt).
So, in my opinion, 3GB is enough for 1440p and high standards of image quality.