ALTER TABLE Table_name
  ADD CONSTRAINT Constraint_name UNIQUE (Column_name) 

USE tempdb;
Go

CREATE TABLE sometest (testid INT, col1 INT, col2 INT);

ALTER TABLE sometest
	ADD CONSTRAINT someconstraint UNIQUE (testid,col1,col2);

INSERT INTO sometest(testid,col1,col2)
	VALUES (NULL,1,1),(1,NULL,1),(NULL,NULL,NULL);

SELECT *
	FROM sometest;
	
DROP TABLE sometest;