MTG Keyword Rules — Comprehensive Rules Glossary

Tripling a creature’s power and/or toughness creates a continuous effect. This effect modifies that creature’s power and/or toughness but doesn’t set those characteristics to a specific value. See rule 613.4c.

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.

If a creature’s power is less than 0 when it’s tripled, tripling that creature’s power instead means that the creature gets -X/-0, where X is twice the difference between 0 and its power. Similarly, if its toughness is less than 0 when tripled, it gets -0/-X. If one characteristic’s value is negative but the other isn’t when both are tripled, it gets -X/+Y or +X/-Y, as appropriate.