I've re-installed my X3 and Scarlett 18i8 on my new computer. The same setup as my last computer (it worked fine), Windows 10 with the same model processor and MB, but X3 isn't recognizing the Scarlett Mixcontrol driver. In fact when I click on ASIO Panel... in Driver Settings it causes X3 to crash. Any ideas?

Make sure Win 10 is reasonably up to date. 
 
Is the MixControl software a new or recent download?  If so, could be a corrupted download.  Try downloading the file again.

Download and install the latest Focusrite driver from the support page.
Install it,
Re boot computer
Turn on the Interface and make sure Windows pops up a notifiction that it has recognizedthe device and it is good to go.
Click on your speaker icon and open the control panel
Make sure the Scarlett is the default audio device and is checked green. 
Now it will work. 
 
What can happen is another audio driver takes control like HDMI audio  so a lot of use disable that in Device manager.