Is S49 mk3 compatibile with osx mojave?

You'll want to find out if Komplete Kontrol and Kontakt are compatible with OSX Mojave. That's the software that really gives power to the MK3.

Otherwise the MK3 is just a MIDI keyboard will work at some level with any MIDI gear/computer.

But to your point, I'm not sure if the current versions of either program support Mojave, or what version you would need for your system. I would do a quick google search for "komplete kontrol mojave" or "kontakt mojave" and hopefully someone who knows more about older version of NI software will comment.