W.Green, G.Swanborough The Complete Book of Fighters

SPAD SA.3 France
  
   While based fundamentally on the SA.2 two-seat fighter, the SA.3 was of highly innovatory concept in that it featured fore- and aft-mounted 7,7-mm Lewis guns and it was intended that either occupant could control the aircraft while the other operated his gun. No record has apparently survived of the geometry of the dual control arrangement, which must have been exceptionally complex, but the difficulty of adequate communication between the two widely-spaced cockpits - with the 110 hp Le Rhone 9J rotary engine and its propeller between - must have proved insurmountable. The SA.3 may have inspired the SPAD Type C, a "pulpit” type three-seater (presumably for a pilot plus front and rear gunners), although it is doubtful that this type was built. It may be presumed that data for the SA.3 were generally similar to those for the SA.2.

J.Herris, D.Mechi The Spad A Series (A Centennial Perspective on Great War Airplanes 104)

Spad S.A.3 & Other Spad S.A.Derivatives

  The S.A.3 was a single prototype with dual controls and a mock-up machine gun for the pilot. It had SFA number S-40 and the manufacturer's number was likely 401. Its airframe was a modified S.A.2 and it was powered by the 110 hp Le Rhone. The nacelle was broadened at the nose to accommodate rudder bars for the gunner and the cooling ducts were not as long horizontally but taller vertically than on the S.A.2.
  The intention of the S.A.3 was to enable either crew member to act as gunner and the other as pilot depending on the position of the enemy relative to the aircraft. There appears to be a voice tube around the lower portion of the starboard under-carriage strut to help crew coordination as was tried on earlier S.A. aircraft.; its effectiveness is not known.
  The S.A.3 was built because Béchereau hoped to quickly make the standard SA design more reliable. Development of the SA, originally planned to last 10 days, was not completed after a month. On 17 September a commission meeting at Chalais stated only that it would "at that point soon". Experience with the SA indicated to the design team and military authorities that the problems would never be solved.
  The S.A.3 was also in response to an official G.Q.G. request for a fighter with more power and guns fore and aft made 2 August 1915. Although the authorities preferred the use of a twin-engine, three-place aircraft, the S.A.3 potentially could solve the desire with a smaller aircraft with a single engine.
  The S.A.3 was doomed by the poor cooling of the engine and difficulty of the crew members coordinating in flight. It finished its life as a trainer.