any over 60 isn't needed, right
not exactly. 60 is where our brain is fooled well enough to consider subsequent images as a fluid non-stop flow, but as you know, real life has no frame limits. Our eyes can see and our brains can handle an ongoing stream of moving things no matter how fast they go. You can easily follow a tennis ball bouncing 150 mph without getting a feeling of it being teleported from one end to another, as well as a jet breaking the sound barrier. Faster monitors make movements look more fluent and life-like.
1. no, Windows is enough given your video card has three outputs.
2. For a three monitor AH experience you'll have to set the video settings accordingly. If you use 1080p monitors in the normal position, the resolution for AH has to be determined to (3x1920)x1080, i.e. 5760x1080. If you turn them on the narrow side, you can set the resolution to (3x1080)x1920 i.e. 3240x1920. That would solve the issue with the aspect ratio FLS mentioned.
3. multiplying the amount of monitors will multiply the load by the same amount. Same goes for adding to the refresh rate. Three monitors is three times the load of one of the same kind. If you currently can get a solid 3x75=225 fps with vertical sync
off (for testing purposes only) on a single 1080p monitor @ 75 Hz, then you should get a solid 75 fps using three similar ones.