Return 1st or last row from NTILE
You could do something like this:

WITH your_mom
    AS (   SELECT   T.ID, CAST(T.ID AS VARCHAR) + ',' AS [FORMATED ID], NTILE(3) OVER ( ORDER BY T.ID ) AS [GROUP]
           FROM     #TEST AS T )
SELECT  *
FROM    your_mom AS b
WHERE   b.ID = (   SELECT   MAX(b2.ID)
                   FROM     your_mom AS b2
                   WHERE    b2.[GROUP] = b.[GROUP] );