Just an extra closing paren.
INNER JOIN cte2 b ON a1.AdmitCCYYMM = b.EffectiveCCYYMM
LEFT JOIN cte1 a2 ON a1.AdmitCCYYMM = SubString(CONVERT(varchar, DATEADD(Month,1,a2.AdmitCCYYMM + '01'), 112), 1, 6)
LEFT JOIN cte1 a3 ON a1.AdmitCCYYMM = SubString(CONVERT(varchar, DATEADD(Month,2,a3.AdmitCCYYMM + '01'), 112), 1, 6)
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How to Post to get the most: http://www.sqlservercentral.com/articles/Best+Practices/61537/