Please check if the system toggle is on the "Windows" mode to match your device. To switch between function and multimedia keys (F1-F12), you can try:
For Windows: Hold fn + X + L for 4 seconds to switch
For Mac: Go to system settings ➡️ Keyboard ➡️ enable the function keys
*For F5 and F6 as function keys, press fn + K + C for 4 seconds and use the same shortcuts to switch.
(Refer to Keychron K3 User Manual for more info.)