وَفِي تِسْعِينَ: ثَلَاثَةُ أَتْبِعَةٍ،
Translation: At ninety cattle, three tabi‘ are due.
90 = 30 + 30 + 30; hence three tabi‘ are due, one for each thirty-animal component.
The progression is not a simple ladder of increasingly older animals at every ten-cow threshold. Instead, the obligation is recalculated using groups of thirty and forty.