Random order at three-column Slick Slider
I already found the solution:

The randomize option has to be enabled in the slick options instead of the view's sort-criteria. 
We can either define a optionset to use it in different views (the optionsets have to be set at Configuration -> Media -> Slick) or we override the default-optionset in the view configuration itself (View -> Edit -> Format -> Settings next to Slick Carousel).