J_T
In the 727 case, I believe the design was done because at the time the FAA imposed some tough take-off weather restrictions on twins, i.e. the DC -9. I'm guessing something like a twin required landing weather at the departure airport, that is, no reduced weather for takeoff. This was especially important to United and Eastern as launch customers operating in the NE US in winter. A tri-jet could depart more reliably.
I'll do some research when back from som recurrent, perhaps a discussion on Vmcg, too
GF