no turns are allowed for in the 3rd segment because of the impossibility of calculating turning radius with changing speed.Once again, based on an engine out.
While some companies or jurisdictions may so dictate, there is no engineering reason why a turn can't be scheduled in the third segment.
Two main difficulties arise -
(a) radius of turn - adjust the splay by min/max speed geometry and that problem goes away very quickly
(b) the AFM generally doesn't give any way to adjust for the additional drag (cf reduction in climb gradient during the other segments) - that has to be addressed by some rational means which, by rights, should also involve local regulatory authorisation.