The answer to all three is whatever you specify, nonclustered by default.
ALTER TABLE ... ADD CONSTRAINT ... UNIQUE CLUSTERED -- creates a clustered index to support the unique constraint. Will fail if there's an existing clustered index
ALTER TABLE ... ADD CONSTRAINT ... UNIQUE NONCLUSTERED -- creates a nonclustered index
Gail Shaw
Microsoft Certified Master: SQL Server, MVP, M.Sc (Comp Sci)
SQL In The Wild: Discussions on DB performance with occasional diversions into recoverability