。
Next year I'll be three times your age.
Literal
Next-year I [topic-は] you [possessive-の] 3-times [possessive-の] age [into-に] become.
~倍 is the multiplier suffix — 3倍 ('three times'), 10倍 ('ten-fold'). ~になる on a quantity expresses 'become [that quantity].' The mathematical claim is a learner-style construction: in practice, being three times someone's age only works in a narrow age window, but the structure itself is grammatically clean.