That really depends on the resolution of the 21x9 monitor. If they are both about the same pixel count the GPU load will be comparable, the overhead from the multiple monitor set up shouldn't enough to make a notable difference.
The 21:9 screens i was finding are 2560:1080 which isn't very high resolution, only about 33% more pixels than a standard 1920x1080 screen, if this is what you are comparing to a 3 screen setup, it would require far less graphics power as the three screen setup would have 2.25x as many pixels.
As for which would be better for gaming, that is really up to personal preference and which monitors you are looking to use for your triple screen setup, skinny borders are recommended.