There are at least three options from within the YN622 system that should work for what you wish to do:
Rather than using a YN622N-TX, you can use a YN-622N II. You give up the LCD screen that makes it easier to see what your settings are, but you gain a hot shoe on top of the YN-622N II to which you can attach a YN685N.
There will be slight differences between the Canon and Nikon implementations (I'm primarily a Canon shooter) so I can't be as precise as I'd like to be about the capabilities of a YN685N stacked on top of a YN622N II. If your D810 allowed controlling multiple flashes from the camera's internal menu (like most post 2009 Canon bodies do), then you wouldn't miss the LCD screen on the YN622N-TX near as much. Since Nikon does not provide that option, Yongnuo has included a limited "display capability" when a YN685N is placed on top of a YN622N II. From Flash Havoc's announcement regarding the YN622N:
At the time of writing this update (August 2016) the YongNuo YN685 N flash provides an “Extended Display Mode”, when used with the YN-622N II as transmitter.
The YN685 N do not directly provide any radio master functions. Though when mounted on top of a YN-622N II transmitter, the YN685 N can be set to Extended Display mode, where the flash then provides a more convenient control panel and LCD display for the YN-622N II transmitter.
YN968N with built-in YN622N Transmitter
You could also upgrade the on-camera flash to a YN968N, which does provide direct radio control of multiple off-camera Yongnuo 622 flashes and receivers, including the YN685N, from the control panel of the YN968N.
This would eliminate the need for a separate radio transmitter altogether. If you're going to be shooting weddings with Yongnuo flashes, it's always a good idea to have a spare available. So you need another flash anyway.
Based on the comments you made to another answer, you could also use a flash bracket attached to your camera to mount the third YN685N and control it wirelessly from the YN622N-TX.
They come in various flavors from very cheap
to very expensive.
Be sure you have the latest versions of the YN685N and YN622N-TX, as I seem to recall that there were issues with some Yongnuo flashes being too close to Yongnuo transmitters at one time.