It was just Naruto, Sasuke and Sakura from the beginning. Now that Sasuke was gone, he left a massive hole in their mutual friendship. The remaining Team 7 tried their hardest to stitch it back up, but no matter how hard they tried, it was always there.
When Sai joined them, the hole was covered by a patch of different material. He covered the hole, but it was obvious that he didn't belong. Eventually, he left, tearing off the patch, and again exposing the gap Sasuke left and the stitches Naruto and Sakura made.
When Sasuke returned, he was sewn back into place. He was a bit different from the others, but he still matched. It was clear that they belonged together.
By the time that he returned, other pieces were joined to theirs. The Hyuuga Heiress was joined to the Kyuubi host, and the Inuzuka and the Aburame were joined to her. Ino-pig was linked to Forehead, bringing with her Team Asuma. The Nara prodigy brought Temari. The Shukaku jinchuuriki was joined to his fellow jinchuuriki, bringing along Kankuro. The Hyuuga genius was joined to his heiress, the junior Green Beast of Konoha with the Weapons Mistress to the idiot. Sasuke fit perfectly among them.
They knew time and time again will cut these stitches. But with care, it will also bring them together.