What if Story of Aircraft:
During the "Battle of Britain" it became drastically clear that the Messerschmitt Me 110 heavy escort fighter did not live up to its expected task of protecting Luftwaffe's bombers. Due to heavy losses inflicted by RAF fighters, Messerschmitt Me 109 fighters had to take over additional protection duties for the Me 110's too, which in turn diminished their availabilty for bomber escort missions.

 

By installing a third engine, it was hoped to achieve a more acceptable performance. In order to test these calculations, some Me 110 C and D airframes were accordingly converted, leading to Me 110 K (Me 110 K-2 based on C airframes and Me 110 K-3 based on D airframes).
Installation of a third engine taken from a Me 109 E (Daimler Benz DB 601 A) resulted in an increase of speed, which was countered by the now higher overal weight of the plane (due to the weight of additional engine and the fuel drop tanks needed).