The Thunderbolt 3 to Thunderbolt 2 adapter does not pass DisplayPort signals. It can pass a Thunderbolt encoded DisplayPort stream over Thunderbolt to a Thunderbolt display, but the LG 43UD79-B is not a Thunderbolt display.
The LG 43UD79-B has a DisplayPort input that you can use instead. For the USB-C port, it may be possible to use a USB-C to DisplayPort cable (I think they're reversible) but you wouldn't be able to use the USB features of the display (which might include audio, unless audio is handled over DisplayPort).
For a display that has only a USB-C input, you can use a DisplayPort to USB-C converter. These don't exist, except as a PCIe card such as the Sunix UPD2018 or Delock 89582 for USB-C with DisplayPort alt mode. A Thunderbolt add-in card such as the GC-ALPINE RIDGE can also work to convert DisplayPort to USB-C with DisplayPort alt mode for USB-C displays, and can also convert DisplayPort and dual DisplayPort to Thunderbolt for Thunderbolt displays. PCIe cards require power from a PCIe slot, but the slot doesn't need to be connected to the same computer to support DisplayPort conversion. If you also want to use the other features of the cards (USB or Thunderbolt controller) then the card needs to be in a PCIe slot that is accessible by the computer. A Thunderbolt card must be placed in a PCIe slot that supports a Thunderbolt add-in card only if you want to use the USB controller or PCIe transmission over Thunderbolt. Conversion of DisplayPort to USB-C with DisplayPort alt mode or to Thunderbolt does not require a PCIe slot that supports Thunderbolt.