Here is the back story behind the development of the Three engine Raptor (A.K.A. "VelociRaptor").
Just before the Cylons broke the Armistice and started the holocaust, Scorpio Military Industries was developing a new type of Raptor designated "VelociRaptor". This new Raptor was adapted with a third engine for greater speed and longer FTL jump capabilities. There were four variants made; MedEvac, Recon/surveillance, gun ship, and troop/personnel transport.
All four of these prototypes were placed on the Pegasus for field trials, while she was in "DRY-DOCK" for refit (and remained on-board after Admiral Cain gave the order to jump away during the Cylon attack).
Shortly after Major Adama too command of the Pegasus, he was told about these new prototype Raptors. Major. Adama had them transferred to the Galactica to finish the field trials and to have them put into service by Chief Tyrol.
All four Raptors passed with flying colours, unfortunately, the Galactica did not have to ability to make any more, so these four were the only ones ever made.