Speak in complete sentences when appropriate to task and situation in order to provide requested detail or clarification. (See grade 3 Language standards 1 and 3 here for specific expectations.)

There's no speaking in fragments in third grade. Okay, maybe there is, but this standard asks your kids to elevate their aural discourse. This standard is super broad, so you can interpret it many ways. In fact, you should cover this one nearly every day. When a student asks you to clarify instructions or asks for more detail: covered. When a student gives a presentation: covered. When students are involved in group work...well, ya know: covered.