Caveat - no 757 background so my comments are generic.
The basic philosophy is a level third segment to accommodate reconfiguration to the final climb.
However, some aircraft accelerate sufficiently OEI to provide a problem with the flap retraction sequence. A modest climb can be used to slow the mismatch down just a little so that the target clean speed and configuration are more easily achieved as required. Provided the profile is as per the AFM, then the takeoff sums are all kosher.
As I recall, the 733/734 had a similar third segment arrangement ?