In theory you could use up to three simultaneously, we may need to adjust/add some code to assign different identifiers to multiple XDMI cards — so you and/or Roon knows what’s what.
There are 3 slots suitable for the XDMI interface. When XDMI is mounted inside the Olympus server these are blocked, but 3 I/O interface cards do fit.
You’d need 2 I/Os, 3 XDMI baseboards, the 3 output options you’d like to use, and 3 I/O interface cards. Obviously this will not be cheap.
For quick comparisons / demos I use 2 I/Os and swap QFSP cables.
Disclaimer: This is not a recommendation or an attempt to sell more I/Os.