Chapter three of A Separate Peace opens with Gene's revelation that yes, Finny had practically saved his life, but he had also practically cost him his life. In other words, Gene wouldn't have even been on the limb and wouldn't have turned around and lost his balance if not for Finny. Here is the opening paragraph:
Yes, he had practically saved my life. He had also practically lost it for me. I wouldn't have been on that damn limb except for him. I wouldn't have turned around, and so lost my balance, if he hadn't been there. I didn't need to feel any tremendous rush of gratitude toward Phineas.
This revelation by Gene demonstrates his capacity to rationalize and to employ defense mechanisms. Rather than feel gratitude to Finny, which implies a subordinate position, Gene changes his perspective to make Finny the bad guy. Gene shows himself dangerously capable of manipulating his own thoughts to make Finny the guilty one in their relationship.
This tendency will, of course, reach its culmination when Gene begins to see Finny as deliberately sabotaging his studies, and when Gene purposely makes Finny fall from the tree.
The realization that Gene has in this chapter (whether real or imagined) is that Finny is deliberately sabotaging Gene's studies.
Finny seems to arrange things like skipping school, being late, and Blitzball to keep Gene occupied. Gene begins to think this means that there is a jealousy or a competition between them to excel at their different strengths (Gene-academics, Finny-sports). Whether this is true or not is up to the reader as the story continues.
Check out the summary, it will give you a few really good ideas.
In Chapter 3 the boys are having an enjoyable time. They involve themselves in swimming, skipping classes, and participate in the Super Suicide Society of Summer. They meet everynight and plan the events. They also participate in jumping off the limb of the tree into the water.
Despite he joy of the summer days war looms ahead and Gene becomes aware of its constant presence. It is a reality for Gene, and it came as his dawning in the chapter. It was the moment in time that was to stay etched in his mind. He senses that adults reaction to males youth is associated to their recognition that one day the young men will be in the military and off to war.
The second important issue within chapter 3 is that Gene mistakes Finny's mischievous activities as a form of grade competition. He is so jealous over Finny that he thinks that Finny does not want him to do well in school.