Constraints like "One of these three clases must be used." are written in form of comments in the UML model right now.
There is no possibility to tranfer these constriction into the schema (and to check this constriction within an resulting XML).
This would be nice to have, probably with using something like OCL.
Initial analysis has indicated that changes would be significant, tool support is not encouraging and further analysis regarding the impact of transfer syntax validation. All this is not feasible in the time schedule for v3.0. Hence - although in general accepted - this issue is postponed.