With one 2560x1440 monitor you have 3686400 pixels, and with 3 2560x1440 monitors you have 11059200 pixels. The graphics card has to work ~3 times as hard. You will get roughly 3x the performance playing on one monitor vs the 3. The GPU is not split between the monitors in such a way that 33% goes to each monitor. If you have more action on the main screen then the card will be working harder on the main screen. Same goes with the RAM, it uses it where it is needed.
Thanks for answering my question