Monitors/triple monitor gaming
Hello! i've decided to go for a triple monitor gaming with my gpu gtx 970, however i might need to upgrade the gpu to get a solid 60fps on high settings (bf4, gta v, the witcher 3 etc) but would there be any performance difference if i use a lower in game resolution than the native resolution triple 1080p monitors?
GTX 970 will definitely struggle with high/max settings in those titles, especially driving a trio of 1080p monitors at 5760x1080. Lowering the per-display resolution (say to 720p each) would improve performance and reduce the load placed on the graphics card, but whether or not it would attain the performance you're after is hard to say (this depends on a few factors, including the rest of the system's specs, specific settings that you're going to set in each game, whether or not you're going to modify the game (e.g. GTA5 and The Witcher games support mods that can increase the performance requirements)). Also note it will not scale in any linear fashion - you may run into a CPU bounded performance limit whereby lowering the resolution any further has no improvement, and the game simply will not go any faster due to what the CPU can offer.
If your goal is full max ultra at 60 FPS (or greater) at 5760x1080 (or greater) you will likely need a significantly more powerful graphics adapter to run the latest-and-greatest in terms of games; the GTX 1080 or Radeon Fury X would probably be more capable, but even then its likely you will experience lag at various points, or have to lower settings/resolution considerably.
For example, here's a review of the Radeon Fury X with benchmark results for GTA5 at relatively high settings:
At 1080p it is able to average just over 60 FPS (without more data it is impossible to say it is always achieving higher than 60 FPS), while the GTX 970 just barely touches 60 FPS (it is reasonable to assume in both cases there are times where the card will be under 60 FPS, and that this is more likely with the GTX 970) - running at higher resolution (e.g. 1440p) hurts performance relatively significantly.
They also benchmarked Battlefield 4,
Which fares somewhat better, but still is not doing "great" at higher resolutions, especially on the GTX 970. The Witcher 3 fares similarly to GTA5:
Ultimately, what you're after may be beyond the reach of contemporary graphics cards, depending on the IQ settings and framerates you're going to demand of the system. Also keep in mind that not all games "play nicely" with multi-monitor, as they may be Vert- and/or have a fixed FOV (or fixed HUD elements) which will lead to various distortions with multi-head. It may be more practical to consider a physically larger single monitor - a big HDTV for example would provide similar size to a trio of 24 or 27" monitors, present the same 1080p workload as one of those monitors to the graphics card, and have none of the compatibility issues of multi-head gaming. This is just food for thought.
If you have further questions, feel free to ask.