You do not have to take all courses in the level at the same time, but it is highly recommended that you take them together. You will learn English better this way because you will be reading, writing, listening, and speaking in English using all materials appropriate for the level. For example, if you place yourself in level 3, you can take ESL 133 (Grammar and Writing), ESL 136 (Reading and Vocabulary), ESL 135 (Listening and Speaking), and ESL 137 (Spelling). There is also the possibility of taking different levels in different skills. For example, you could be in ESL 133 (Grammar and Writing level 3) and take a different Listening/Speaking class like ESL 145.