I understand that on some types you can afford a slight climb on the third segment. The selection of MCT at the end of the third segment is making the asumption that thrust above MCT was used at the start of the take off. The 10 minute power limitation comes in to play (used to be 5 minutes). MCT must be set at the 10 minute period and the end of the third segment must be complete (ie flap retraction). This was the major reason for a flat segment to make sure you dont exceed the power limitation with flap still out.