DECLARE @i VARCHAR(1)

SET @i = LEFT(NEWID(),1)

SELECT CASE ISNUMERIC(@i) 
            WHEN 1 THEN 
                CASE WHEN @i%3 = 0 THEN '1' ELSE CONVERT(VARCHAR,@i%3) END 
            WHEN 0 THEN 
                CASE ASCII(@i)%65 WHEN  0 THEN 'a' 
                          WHEN  1 THEN 'b' 
                                                  ELSE 'c' END 
        end