As others have said, Collins is laying the foundation for the rest of the series in the first four chapters. There will be many changes to this love triangle as the plot thickens, but in the beginning Katniss and Gale are hunting partners who discovered each other quite accidentally. Peeta is just someone she doesn't even casually know, but they may or may not have shared a special moment. He did something amazing for her, but she isn't sure if he did it on purpose or not.
The beginning of a book is called the exposition. In this case, the relationship between Katniss and Peter seems minor but they will become more important later. This is also a form of foreshadowing, because we are given information about characters before we know their role and full impact on the story.
It is clear that Katniss has a closer relationship with Gale, and barely knows Peeta. However, Katniss does seem to be rather ambiguous about her relationship with Gale. She speaks of him as being a kind of older-brother figure, but at the same time seems to want to deny that she might have any romantic feelings for him and also to ignore any signals that he might be sending her.
Gale is her hunting partner and Peeta is an acquaintance from town who gives her a little extra food from his father's bakery. Katniss and Gale both secure extra food for their families by hunting in the forbidden areas and they teach each other the tricks of being sneaky and resourceful in regards to not getting caught which are skills that will prove more useful as the novel progresses. Peeta's kindness and attention to Katniss will play a larger role later in the novel.
Correct me if I am wrong, but Katniss met / encountered Peeta before she met Gale. He bacame the "boy with the bread" before she met up with Gale in the woods.
Notice the synbology, Katniss associates, accidentally, Peeta with a dandelion - which is very closely linked to her beloved late father. She and Gale also share an inseparable bond since both their fathers were taked with the same event in the mines and Gale is the only person outside her family she trusts - she even leaves her family in his capable hands.
After Katniss made the connection of the dandelion, with her father and "the boy with the bread" she had a new determinantion for survival, giving birth to her rationale and stubborness, which she used for sustainance, leading her to the woods, finding one so similar as herself - Gale.
Katniss at 17, has not really paid much attention to romantic relationships besides, at this point in the books, she has other pressing things to worry about - like her own life, and her family - and her District, a lot on premature shoulders.
Gale did not volunteer in place of Peeta at the Reaping. Maybe he knew that she would place her frail family in his hands so he had to remain in District, which obviously proves useful, or maybe he was scared. However, I am still curious as to what he was trying to say to her just before they parted "And remember I--" and Katniss never found out. I thought he was going to say "I love you".