I have the same setup, and unfortunately I have no solution to offer. I contacted DJI support; the first person I talked to said there was no firmware available for the Matrice 100 and Z3, but that their developers were working on it. The next day I called again, and this person had no idea if developers were even working on this issue, but suggested that I take my Z3 off my Matrice, go find an Inspire and connect it to that, insert the SD card with the Inspire/Z3 firmware, and update the firmware that way (????). Sorry, but I don't happen to have a spare Inspire lying around for the sole purpose of firmware updates for my Matrice. This is a serious problem and I'm hoping DJI is addressing it. Also, I couldn't get the Assistant 2 software to recognize my Matrice 100 (it would just show the USB plugin image). Did you have to do anything special for that step?