I’m using the Apple Digital Multiport Adapter that I already had from my 13” MBP.
I’m really pleased with it so far. I have it hooked up to power via the included USB-C charging cable and AC adapter. HDMI to my Dell U2515H. I have a Anker USB 3.0 hub plugged into the USB-A port, and then my AudioEngine A2+ reference speakers plugged in via USB to the Anker dock, iPad Pro 12.9 gen 3 is supporting the sound out via dock connector just fine!