The Pro 3 sequencer I believe doesn't have song mode.
That all being said it could easily be the centerpiece of a setup. Have the Tempest as the master clock doing a beat hooked up to the Pro 3 and have the Pro 3's sequencer sequencing internal sounds and Moogs, ARPs, Roland System 500 or whatever.
Wow...I just realized something. A big feature request of the Tempest was to be able to sequence more than one mono synth but maybe the Pro 2 and now the Pro 3 was meant to be the piece of gear to do that all along.
As you pointed out, the Pros don't have a song mode. So exactly how are you going to have it sequence other gear beyond the 64 step limit? Unless there's someway around that, I don't see how it could be the true pièce de résistance of your setup... Also, the synths you named are mostly (aside from the One/Memory) mono synths. Your Tempest can already sequence those if it sacrifices a track/voice of its own. The same is true for the Rytm as of the latest update. Do the Pro 2/3's tracks send more than one midi note? If not, then again, you have to awkwardly play one note into it at a time, except you now have the aforementioned step limit. Hopefully I'm simply misinformed and there's a way around the limit and the song mode from some external gear can somehow have the Pros send note information beyond the 64 steps...
Tempest does not have CV capabilities. It has a single MIDI channel for ONE synth voice. So you can only do one external mono synth at a time or layer multiple synths on the same synth channel but again only one channel at a time.
With the Pro 3, although it’s 64 steps may seem limited it depends entirely on the type of music you are doing. Have one track doing one sequence melody going out to say an ARP Odyssey, another going to a Roland System 500 and another going to an Oberheim SEM Pro. Yes, all mono tracks but with the sequencer having the ability to have different length tracks, the ability to transpose the sequences, mute steps, mute tracks, add ratchets all on the fly. And the ability to alternate between 4 sets of sequences with The A, B, C, and D buttons (Intro, verse, chorus, bridge). Plus now you can mix all the sounds of your external gear with the great internal sound of the Pro 3. You literally have a perfect hub for a Klaus Schulze or Tangerine Dream set up. At least for the rhythm and melodies and have some poly synths on the side to do pads, leads etc.
It’s not going to be extensive like a Pyramid, MPC etc but what it could be is a hands on interaction and improvisation of multiple sequences which is what Berlin School essentially is.