I’d like to avoid my computer. Anyone out there that use any of these three mentioned devices with your Zynthian to make your songs? If so i would love to know how the experience is for you!
I am using an Arturia KeyStep Pro. Not the same as you mentioned but also with 4 tracks of step sequencing.
Would be happy to own a Hapax 
Are you thinking about using Zynthian as a basic musical instrument that plays a note in response to a keyboard, MIDI message, etc? Or did you plan to use Zynthian Sequencer or SooperLooper etc in combination with an external sequencer? In my experience either setup can work well, but using Zynthian non-instrument functions with external sequencing gear will sometimes take some effort to set it up as you like. I remember people on this forum having questions, mostly re sync about all 3 brands you mentioned, so a search on the brandname-model might give you some useful insight.