I've bought the Apple Thunderbolt 2 to Thunderbolt 3 adapter, and already have a spare USB A to USB C which I was given by a family member.
Ideally I need an Ethernet one, too. I'm toying with either buying the standalone Belkin adapter, or going with some kind of hub, but the latter seems tricky to find. Minix and Anker both do one, but I know someone who owned both and the USB C connector to the laptop was very loose on both. OWC and Caldigit do ones with probably better build quality, but are very expensive and require mains power. Not really sure if there are any other alternatives: all I really want is a well-built hub with Ethernet, USB A, and maybe passthrough charging, but that's less of an issue on the MBP than it is on the rMB.