How can I make a unique constraint on multiple column in MSSQL

How can I make a unique constraint on multiple column in MSSQL

create table tImprumuturi
(
ID_Imprumut int identity primary key,
DataImprumut smalldatetime default getdate(),
DataScadenta smalldatetime,// here i have to make a function to
DataScadenta=DataImprumut+2
CodCD char(10) foreign key references tCD(CodCd)not null,
CodCV char(10) foreign key references tCaseteVideo(CodCaseta),
CodAb char(10) foreign key references tAbonati(CodAbonat) not null
)
How can I make a UNIQUE constraint on all 3 CodCD, CodCV and CodAb?