OK OK, I can't stand it. the 757-300 DOES NOT have the 'flip up' over wing exits like the 737NG. Trust me, I helped build the first 10 or so that went out of the door.
The #1, #2 and #4 doors are all different. #1 L and #2L are slightly larger as these are mainly used for boarding. #1R is shorter than #1 L as this is used for catering. #2R is about the same as #2L except opposite. and #4L & R are the about same except opposite (and narrower than #1 and #2).
The #3 Emergency door is located in an area of high fuselage skin loads. I like my little science experiments so here goes:-
Example:- Place a ruler hanging over the edge of your desk and apply load to the free end. It will break the ruler at the desk, not mid way along the ruler. This is the same configuration that occurs in the tail section of an aircraft. Add to that torsion induced by the vertical tail loads and hey presto, high stresses!.
Based on this, the bigger the door at that location, the more beefing up of the structure would be required to accomodate the fuselage skin cutout (thicker skins and extra doublers = more weight and more complex structure).
Also to quote from above: "This also meant that no other manufacturer would have to be involved because all 3 types of doors were already used for the 752." (Aq737)
Boeing manufactures the doors for the 737 and 757 (unless it's changed in the past 3 yrs). Some at Witchita and they used to do some at Renton, though this may have changed. Supplier of doors is not a consideration when determining how many and where to install them.
RE: Flip up doors.
I doubt any operator would willingly update their fleet to incorporate flip up emergency exits. The only reason the 737NG has them is because the CAA mandated it in a revision to their regulations (after the Manchester runway fire accident). The 757-300 has a different passenger to door ratio and therefore does not need to comply with this regulation. This would be very labour intensive to incorporate in the field. We had to scramble at Boeing to get the first 20 something aircraft modified before delivery to accomodate this new requirement, loads of overtime though!
Phew, good to get it off my chest.