جَاءَ ثَلَاثَةُ طُلَّابٍ says that 3 students came; ثَلَاثَةُ is an ordinary number here.
دَخَلَ الطُّلَّابُ ثُلَاثَ means that the students entered 3 at a time; ثُلَاثَ is the transformed distributive adjective.
A similar number meaning does not make both forms grammatically identical.
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Cause of Ghayr Munsarif: Transformation (Adl)
جَاءَ ثَلَاثَةُ طُلَّابٍ says that 3 students came; ثَلَاثَةُ is an ordinary number here.
دَخَلَ الطُّلَّابُ ثُلَاثَ means that the students entered 3 at a time; ثُلَاثَ is the transformed distributive adjective.
A similar number meaning does not make both forms grammatically identical.