When you say USB3, it can be one of two types of connections depending on the cutter itself:
A. a true USB connection, like a printer, in which the connection is identified as USB_Printer_3 (in which case it will NOT show up in SCAL's menu of choices but SHOULD still work)
B. it can be a USB-serial connection using an FTDI type driver in which it's identified as a COM(3) connection. Again, it should work with just Auto selected.
I would expect ArtCut to show you the correct labeling so that at least that question will be answered. One thought, based on my own experience with several other cutter brands. If I have made a cut from another program like ArtCut, then I won't typically be able to then get SCAL to communicate with that same cutter. I have to close both programs and restart my cutter. So, maybe try again from a fresh reboot of the cutter and, just to be sure, a fresh reboot of Windows to see if it makes any difference.
Also, be sure click the refresh (circle arrow) icon and then click the Test Connection button with your cutter turned on and in Online mode. If you were testing by having an actual shape cut out, make sure you see the cut in the preview window on the right side in the Cut Settings window.
The other issue is that under Cutter>Cutter Settings, the Baud Rate needs to match what is set on the cutter. Typically, however, having the wrong baud rate does NOT result in a Plotter Not Found error, but rather nothing at all happens.
If none of this helps, then I suggest submitting a support ticket with Craft Edge (the developer of SCAL) using this link:
http://craftedge.com/support/support_form.php