You cannot accumulate dice for victory points over multiple turns. All dice used to score for three-of-a-kind and any additional matching dice must be rolled at the same time, during a single turn.

This is supported by the following rules:

• On your turn, you may roll the dice up to three times, setting aside and re-rolling as you wish, but all dice are resolved at the end of your turn (page 4).
• "If you roll three-of-a-kind of Faces for dices (King of tokyo) (2 kinds of dice, black dice and green dice), gain as many as the number. Each additional die rolled with the same face gains you 1 additional." (page 4)
• There is no mention of accumulating dice results across turns; all dice are resolved at the end of your rolling phase in a single turn.

Summary

All dice used to score for three-of-a-kind and additional matches must be rolled and resolved in the same turn. You cannot accumulate matching dice across multiple turns.

Source: page 4.

Source: Core Rulebook (pp. 2, 3, 4, 5, 6)