I think the first couple of parts have important plot points which lead to the third part. It certainly helps watching it all at once like here: http://www.bnwmovies.com/combat-america.html

Gable is supposed to have flown as a gunner on five missions before they made him stop. He resented being treated as a star and not being allowed to participate as a gunner more.

After his wife, Carole Lombard, was killed in a plane crash, a grieving Clark Gable couldnt face Hollywood anymore, so he joined up with the Army Air Corps, requesting that he be sent into actual combat. So naturally the Army decided he should make a movie about his combat group, the 351st Bombardier Group. Gable narrates this complete portrait of his outfit and appears in several short scenes where he interviews his comrades-in-arms. The focus of the film, though, is the group itself, especially the gunners. The film starts just after training is complete and the group flies to England to begin flying actual missions. The film covers all aspects of the lives of the gunners, from actual combat to leisure time, and from goofy hijinks to the gritty reality of seeing your buddies get killed or seriously wounded in combat. The film is quite similar to The Memphis Belle, though perhaps not as engrossing as that film. Still, the film has quite a bit of historical value as a portrait of combat and army life, and in color to boot.
Ratings: Camp/Humor Value: **. Weirdness: **. Historical Interest: *****. Overall Rating: ****.

In this highly romanticized flick about air bombers, air fighting footage is mixed in with obvious renactments (by the soldiers themselves!) of life at war. Narrated by Clark Gable, who narrated this film as his contribution to the war effort (lucky guy) the hour long film drags a lot, and that's a shame, as it's really cool to look at WWII in color. The whole thing doesnt REALLY get going until the third part, and it's doubtful people would want to stick it out to get to that point.