ثُلَاثَ means “three by three,” describing the distribution of a group, not merely counting to three.
دَخَلَ الطُّلَّابُ ثُلَاثَ
Translation: The students entered in groups of three.
دَخَلَ: past-tense verb. الطُّلَّابُ: nominative subject. ثُلَاثَ: accusative ḥāl (circumstantial qualifier), with fatḥa and no tanwīn.
The complete sentence is an instructional example; the source text itself lists «ثُلَاث».