you can control 3 devices with MX12 but note to have different midi channels for each device to avoid any conflicts.
the problem with value jumps can be solved when the devices send feedback data by changing projects.
for ELEKTRONs this works afaik only with OT. not sure how the tetra works.
so you would have these jumps after changing projects on A4 and RYTM.
a little help is to use the fader mute function on MX12 to move the faders near to the new value (holding shift and move fader).
I hope the ELEKTRON guys will implement this feature (feedback data by project change) in the A4 and RYTM too like on the OT. then you can use the snap function on MX12.
or something like the snap/relative function on ACCESS VIRUS would be a good solution too.
with motorfaders and encoders you would have the same problem because they also need feedback data and encoder relative mode isn’t supported by the ELEKTRONs 
so the main issue is on device side and not on controller side