You don't seem to understand. Kakashi was teaching all three. That's also what was happening up until the Chunin Exam arc. During the first two arcs, Kakashi tested them, then trained them by taking them on a mission to give them experience, and he taught them chakra control during the mission. After the mission, he let them enroll in the Chunin Exams. Naruto and Sasuke were the only ones that passed. Kakashi's obligation, then, was to prepare them for the second half by training them. However, time was limited. Kakashi knew his style didn't fit Naruto; this wasn't a time for general training, this was a time where he needed to enhance a specific strength of theirs. He could do that for Sasuke better than he could for Naruto because his style compliments Sasuke. Kakashi had a choice, focus on Sasuke and have someone else who is better suited train Naruto for that occasion, which would benefit them both, or train them both at once, which would result in Naruto receiving adequate training and Sasuke's benefit being squandered. Style compatibility was important for this occasion, the only occasion where Kakashi singled out Sasuke for training. Afterwards, Sasuke got the Curse Mark, and Kakashi was obligated to keep it in check for Sasuke's safety. Afterwards, he trained them together again, but then Sasuke left. After Sasuke left, Sakura went to Tsunade and Naruto went to Jiraiya.
