Apple Thunderbolt 3 to Thunderbolt 2 cable -> 34UM88 -> Thunderbolt 2 to Thunderbolt 2 cable -> 34UM88.
To make it even more fun, Thunderbolt 3 looks like USB-C and Thunderbolt 2 looks like Mini DisplayPort.
https://i.imgur.com/5TJaIqg.png
(Technically I do have a dock but it's on the other side and is unrelated).
What do you mean? Thunderbolt 3 uses USB-C connectors. And you can also use a Thunderbolt 3 cable as a USB 3.1 Gen2/DP-Alt/PD cable.
The big annoyance is that it doesn't work the other way around. There are even USB-C cables that only carry USB 2.0 and do not have the SuperSpeed lanes. So, they are not only worthless for Thunderbolt 3, but also for getting good tranfer rates using newer USB revisions. Worst of all is that there is no cable marking system.
At least usually you'll get something that kinda works.