Parts C are glued onto parts A and B, If it appears that Part C is slightly higher or smaller than Parts A or B, make sure that the bottoms of Parts A & C or B & C fit perfectly. The upper part can be ajusted later on by sanding them to the same level (the discrepancies however should be in the order of a few tenth of mm). I advice to use adhesive tape to keep the parts together waiting for the glue to dry.