Rule 701.11b

To triple a creature’s power, that creature gets +X/+0, where X is twice that creature’s power as the spell or ability that triples its power resolves. Similarly, an effect that triples a creature’s toughness gives it +0/+X, where X is twice that creature’s toughness. Tripling a creature’s power and toughness gives it +X/+Y, where X is twice its power and Y is twice its toughness.