Do you record in single speed, 44.1/48 kHz?
Then you can connect two of your 32ch interfaces to one MADI Bus and the bridge is not needed.
You have even one MADI bus free for other purposes.

XT MADI Bus 1:  device#1: ch 1…32   device#2: ch 33…64
XT MADI Bus 2:  device#2: ch 1…32   device#4: ch 33…64

If you record in double speed, then only 32 channels are available per MADI bus.
Then you require the MADI bridge to be able to patch any three of the four AD/DA converters to the 3 MADI busses of the XT.
Use optical ports where possible to have also galvanic isolation.

Connect the AD/DA converter to the inputs (1-6: BNC, 7-8: optical)
Connect the MADI bridge outputs to the XTC
       MADI bridge output #7 (optical) -----> XTC MADI Bus #1
       MADI bridge output #8 (optical) -----> XTC MADI Bus #2
       MADI bridge output #6 (optical) -----> XTC MADI Bus #3

Then on the front of the MADI bridge, select MADI Source / Destination:
      For output #7 (XTC MADI Bus #1) select “input channel for AD/DA converter #1”
      For output #8 (XTC MADI Bus #2) select “input channel for AD/DA converter #2”
      For output #6 (XTC MADI Bus #3) select “input channel for AD/DA converter #3”
For the 4th AD/DA converter you need to check when one of the others three MADI busses at the XTC is free and patch it to either output 7, 8 or 6.

See also old product page, from the description and the front panel/back the functionality should be clear.
https://archiv.rme-audio.de/en/products/madi_bridge.php

If you need a converter between MADI optical and BNC, then search for this product on used market:
https://archiv.rme-audio.de/en/products … verter.php