DVDdoug's advice in post #2 is generally correct, but let me add some additional info -
portable amp/speakers such as your M3 usually have a
digital amplifier, which is a great idea because they include DSP circuitry to do digital crossover for the woofer/tweeters. Also this means the connection from wifi/bluetooth receiver unit is pure digital - there's a saying the best DAC is no DAC.
There's just one downside - the "AUX" input is analogue, and it must go through an ADC chip before the amplifier, and usually these ADC circuits are cheap and nasty ...
so for best quality with portable amp/speakers you need to utilise the inbuilt wifi receiver,
not the AUX input.
OK, so the question is how to get audio from your FiiO X5 III to the GGMM M3 via wifi?
First connect both the X5 and M3 to the same wifi router. If you don't have a wifi router in range, set the M3 in "access point" mode by pressing the RESET button for 5 seconds, then connect the X5 to this wifi network.
then option 1: on the X5 install the "GGMM-M Series" app on your X5. It should provide some form of wifi connectivity for your music library to the M3.
or option 2: the M3 can act as a DLNA endpoint, so on the X5 install "BubbleUPnP" and configure it as a server. Basic instructions here -
https://www.makeuseof.com/tag/turn-old-android-device-media-server/