Hello!
Is there a way to show in game FPS? A command or something, or do you need a 3:e pary program for that?

I haven't found a way via the game itself. You can grab the free version of bandicam or fraps. They will show FPS while playing. Also afterburner and others.

If you have an NVidia card you can turn it on through Geforce Experience.

Preferences Tab->ShadowPlay on the left->FPS Counter box on the right

Just having an Nvidia card is not enough. You have to meet some minimum spec requirements for shadowplay.

Well sure, but if you are playing Witcher 3 you probably meet those requirements which are:

Operating System:
Windows 8, 8.1
Windows 7
Windows Vista (DirectX 11 Runtime required)
Windows XP SP3 (driver updates only)

RAM:
2GB system memory

Disk Space Required:
20MB minimum

Display Resolution:
Any display with 1024x768 to 3840×2160 resolution

Supported Hardware:
CPU:
Intel Pentium G Series, Core 2 Duo, Quad Core i3, i5, i7, or higher
AMD Phenom II, Athlon II, Phenom X4, FX or higher

GPU Series
GeForce GTX class GPUs 900, 700, 600,
900M, 800M, 700M, select 600M Series