the simplest way to using that setup is replace the US1800 with a soundcard with ADAT I/O.
It's absurd to use 3 interfaces!
Do you have firewire input in your computer? Or only USB?
you can take a look to motu hybrid interfaces, so you can have both firewire and usb.
for example Motu 896 Hybrid (8 analog input+16 digital channel-->2 adat I/O)
With ADAT you can have 8 I/O channels for each cable, or 4 if you work at 96k so your setup will be more easier than the one you thought:
8 (or 4) input octopre---->adat out
adat it--->new audio interface (es. motu)
so you will have 8 (or 4) digital channels from octopre, and all the analog input of the new interface and the second adat channel free (you can think to buy another octopre to use it with it), free to use them as you prefer!
In this way, with one audio interface you will have the biggest numer of channels