The Microham USB III works fine with linux and fldigi.
Under Windows their USB Router software provides a convenient interface to the USB III to assign functions (like PTT) to virtual COM ports and set audio levels.
Under Linux, the USB III appears as /dev/TTYUSBX, where X is the number of the USB-serial port and as a sound card.
There is a linux configuration issue about using USB-serial devices related to permissions and the "dialout" group which is not specific to the USB III.
From the USB III manual:
USB Interface III is compatible with most current LINUX distributions. It is supported as standard USB
Serial and USB Audio devices. USB Interface III contains circuits specifically to support features of FLDIGI
) by Dave Freese, W1HKJ.