Are there still 3 Mixers?

I have pretty much always had 3 mixer windows.
Number One is my Audio and VST Instruments.
Two is my Submixes and Fx.
3 is my Inputs.
I use F3, Alt-F3, and Ctrl-F3 respectively.
Has this option been removed from Cubase 10?

No, it’s the same.

Still the same, but more flexible and quicker to use just 1 and have key commands for different mixer configurations, assuming you only need to view one mixer at once.