That’s good-going if you have only had the OT for a month!
So far, so good!
As you learned when you were using just the DT and DN, the MIDI Program Change messages that the Elektron gear uses for this purpose is sent on a specific MIDI channel.
On the OT this MIDI channel number is set in the PROJECT > MIDI > SYNC menu. The setting is labelled “Ch”, immediately below PROG CH SEND on the screen. I recommend that you set it to a specific number, for example 16.
In the DT and DN you set the MIDI channel number for reception of these messages in their SETTINGS > MIDI CONFIG > CHANNELS menus (the same for both instruments). Set PROGRAM CHG IN CH to the same number (for example, 16) that you used on the OT.
That should fix things: check it out and report back.
The OT by default sends out MIDI messages and the DN and DT receive them. But that can be changed.
In the OT’s PROJECT > MIDI > CONTROL menu, set AUDIO CC OUT to “INT”.
That alone should be sufficient in most scenarios. But you can make doubly sure that there is no unexpected behaviour by adjusting the behaviour of the DT and DN. In the SETTINGS > MIDI CONFIG > PORT CONFIG menu on both instruments, disactivate RECEIVE CC/NRPN.
I know. You have three VERY powerful machines, and it does take some time, some manual reading, and asking some questions to get where you want.
There may still be more tweaks to make as you refine your setup. I recommend that you save your Projects on each machine as you tweak them, to use as template projects in the future so that you don’t need to set up all the settings again each time you begin another project.
We’ll be here to help.