You dont know what you are talking about.
The reason we bring up 4k displays is because of pixels. A 1080p monitor has about 2 million pixels. A 4k monitor has about 8 million pixels. If you take 3 1080p monitors and put them in surround in a game, you get 6 million total pixels. This is why we are comparing it to 4k. Because 3 1080p monitors in surround in a game has about 25% fewer pixels than a 4k panel. This is also why it was referred to as 3k. But I digress.
Single slot has ABSOLUTELY nothing to do with the card. Everything that is needed on a GPU can easily fit on a PCB and does not even require a full slot. What uses the extra slots is cooling. The chip, VRM, and memory on the PCB can get hot. Heat hurts performance. So manufactures put coolers on video cards. The same way that you put a cooler on a a CPU. Except these coolers already come pre-installed. 90% of a GPU is the cooler.
Power has nothing to do with it either. You can have a single slot card run with additional power provided by the PSU that is required beyond the PCIe slot. These cards are not common because more power creates more heat. Therefore you need a good cooler. Coolers are big and they make them more slots.
Lastly, yes, the cards can support 4 displays at 60hz. If you hook up the 4 displays and load into windows you will be able to go into the control panel and they will run at 60hz. But that is not what the OP asked.
He asked:
Will i be able to play on 60 fps with 3 monitors at least on high or medium details? (i want to use all 3 monitors for gameplay on games like Witcher 3, Rise of the Tomb Raider etc.)
The answer to that question is a no. It is one thing to boot into windows on a 2d environment and run 3 dispalys at 60hz. It is a completely different thing to run those displays during a game and expect 60fps. The OP asked about 60 FPS at high or medium details.
You may ask yourself, how do I know that it wont work. I dont have a 980 and 3 1080p panels. But i do know. Because a 980 cant run The Witcher 3 at 4k and get close to 60fps. And since we are talking about pushing 25% fewer pixels than 4k, it is a very close comparison.
Hopefully you will take the time to actually read this. You might even learn a thing or two. But I suppose you wont, Bro...