Unique constraint on multiple nullable columns Sql Server
There's nothing special that would prevent you from using a standard unique constraint here, unless there's something you aren't telling us.

When a unique constraint is created on a set of nullable columns, NULL is treated (in terms of uniqueness) like any other of the distinct values in the set. (I believe this behaviour varies across RDBMSes.)

So I think the only case you might have to protect against here is the where all three columns contain NULL at the same time. If this is invalid for your business logic, also create a CHECK constraint to disallow this combination.