Is there some sort of clever hack of achieving that, because I did not find anything. I have a 27" Mac and everything is still kind of huge.
No - you get two windows in the current implementation. One on the left and one on the right.
I suppose the notification center slide over could be considered a "third window" but you can't put an application window there - it's just a system tray of sorts.
I would look at a program like Divvy on the Mac App Store for customizing a workspace with multiple tiled windows. It's not split view, but it's far more flexible and able to make use of the 27 inch display space to quite a granular basis.
You can use Rectangle to achieve this for free.
Rectangle is a window management app based on Spectacle, written in Swift.
After downloading, installing, and granting the app accessibility permissions in System Preferences, you'll be able to enable the default keyboard shortcut scheme, which allows you to press ctrl + opt + U/I/K/J to snap a window to the top left/top right/bottom right/bottom left, respectively. By assigning four windows each to one of these four positions, you can create a four-way split screen view (and if you enable Dock and Menu Bar hiding, you might be able to get a bit more screen real estate, if needed).
Hopefully something like this becomes a standard Mac feature. I like the split screen feature, but sometimes I want/need more than two screens. I found a pretty good solution on the Apps store that will let you have split screen, one window half/two in quarters, or 4 windows is quarters. It is called Magnet. Works great on OSx Sierra, and is much cheaper than Divvy. Here's the link!
If you want a more powerful alternative to Rectangle, you could use the free and open-soure Yabai, which you can configure however you like.
yabai is a window management utility that is designed to work as an extension to the built-in window manager of macOS. yabai allows you to control your windows, spaces and displays freely using an intuitive command line interface and optionally set user-defined keyboard shortcuts using skhd and other third-party software.
An app you might want to check is BetterSnapTool. It does what you want and is available from the Mac App Store. It sells for $2.99. I know you wanted a free app but I couldn't find one for the Mac.
BetterSnapTool allows you to easily manage your window positions and sizes by either dragging them to one of your screens corners or to the top, left or right side of your screen. This lets you easily maximize your windows or position them side by side. In addition, you can set keyboard shortcuts in order to move and resize your windows the way you want.
@Harry is actually providing a very good recommendation without being very specific about it.
Spectacle does (almost) exactly what you're looking for. It's not strictly speaking split view, but it allows you to split windows between the three third of the screen via simple shortcuts:
In addition, it's free and open-source.
SizeUp is a window manager app like the others. It costs $12.99 at the time of writing, but offers unlimited free trial (it just reminds you every now and then to pay).
SizeUp allows you to quickly resize and position your windows with keyboard shortcuts or a handy menu bar icon.
It is not open source, though.
Me from the future here; I ended up settling for Magnet
It has nice logical shortcuts to snap windows to quarters, halves and thirds of the screen and supports dragging to the edges of the screen, like in Windows. Easily worth the 1.09€ for me.
Ended up switching to Divvy. It allows me to place windows anywhere on a customizable grid (kind of like a less aggressive tiling window manager). And I can define custom shortcuts. So I took Magnet’s shortcuts and added a whole bunch of other ones, like centering, top left sixth, bottom quarter, etc.
It also allows to set a custom window separation, which I think looks pretty good.
BetterSnapTool has worked well for me for years