Regarding the question that asks what the girls were wearing, that question can be answered by reading the very first sentence of the story. The three girls are wearing bathing suits. Sammy is the story's narrator, and he specifically tells readers that the first of the three girls that he noticed was the "chunky" girl that was wearing the plaid two piece bathing suit. Sammy also notes that the girl has a good tan and an attractive butt with some tan lines just underneath it. He will also eventually tell readers that he suspects she just got the suit because her belly isn't quite as tan as it should be:
It was bright green and the seams on the bra were still sharp and her belly was still pretty pale so I guessed she just got it [the suit].
It takes Sammy a bit to get over his shock that three teenage girls in bathing suits have walked into his store, which is not close to the beach, but he eventually gets over enough of his shock to tell readers that the three girls were not wearing shoes either.
The second girl is the girl that we get the least detail about. Sammy notes that she is tall, has black hair, and she has a chin that is "too long." The third girl is by far the most attractive girl of the bunch, and Sammy nicknames her "Queenie." Readers are told that she has "long white prima donna legs." She is wearing a light pink or beige suit with the straps hanging off of her shoulders. She has "oaky" hair that the sun has started to bleach out. She wears her hair in a bun and has a "prim face."