Based on my testing, it appears that what you propose is a somewhat good indicator of how triple screens will perform, but the reduction is not quite strong enough. My results were (I dropped the Min values as they're basically meaningless):
Single monitor:
2560x1440 resolution, 'Single Screen': Avg 98, Max 128
2560x1440 resolution, 'Triple Screen': Avg 83, Max 106 (15% drop)
Triple monitors:
7680x1440 resolution, 'Triple Screen': Avg 77, Max 98 (21% drop)
You saw a drop of 21% going from 'Single' to 'Triple' screen on your single monitor (119 to 94), so my best guess based on above is that maybe you'd see a total reduction of 1.4 times 21%, or 29% and thus an average FPS of something like 84? Obviously still quite playable, so good news for you!
I'm on an old i5-2500k (overclocked to 4.8GHz) and a GTX 1080Ti.