So would it be safe to daisey chain the Sunn cab with a Marshall cab?
That would have an impedance of 12 ohms, so no.
Actually that would have an total nominal impedance of of 4 ohms, so yes, it would be fine.
To run all three cabinets at once, you could daisy chain the two Marshall cabs together using a parallel Y cord. Plug the Y into one of the outputs on the head, and the Sunn cabinet into the other.
This would accomplish exactly the same thing as daisy chaining the three cabs together in parallel.
Any way you can connect all the cabs in parallel will work as long as the is a plug in the 4 ohm jack on the Sorado.
I would amend this absolutely correct statement to read "Any way you can connect all the cabs in parallel will work as long as the whole mess is connected to the 4 ohm tap on the Sorado."
Here's the rub; Each 4X12 Marshall cab will only be receiving 25% of the amp's power, while the 2X12 will be recieving a full 50% share.
Probably not the most efficient use of that group of cabinets IMO, but to each their own!