Store Procedure Prob?

Hi,
I have three different tables. I want one login screen and check all table ,if any user got it,but i have written simple store procedure and i take values by parametrically,how can i know user's table name?
ALTER PROCEDURE [dbo].[sp_GirisKontrol]
    -- Add the parameters for the stored procedure here
    @Username nchar(12),
    @Password nchar(12),
    @Id int output,
    @Rowguid UniqueIdentifier output
AS
BEGIN
    SET NOCOUNT ON;

    SELECT @Id=Id,@Rowguid=Rowguid FROM Admin WHERE Username =@Username AND Password =@Password
UNION
    SELECT @Id=Id,@Rowguid=Rowguid FROM Ogretmen WHERE Username =@Username AND Password =@Password

UNION
SELECT @Id=Id,@Rowguid=Rowguid FROM Ogrenci WHERE Username =@Username AND Password =@Password
   
    if(@Id IS NULL)
    SELECT @Id=-1
END