To assess the effect of adding acarbose or repaglinide or gliclazide to sitagliptin plus metformin, compared to adding glimepiride, on glycemic improvements in Type 2 Diabetes Mellitus (T2DM) participants who require the addition of a third oral anti-hyperglycemic agent (OAHA) according to China Guideline for Type 2 Diabetes. The three co-primary hypotheses are that after 24 weeks of treatment in phase 2, the mean change from baseline in hemoglobin A1c (A1c) in participants receiving either (1)acarbose or (2)repaglinide or (3)gliclazide added to sitagliptin and metformin combination is non-inferior to that of participants receiving glimepiride added to sitagliptin and metformin combination. The study would be declared successful if at least one of the three primary hypotheses was met.