In cresol both $+I$ effect as well as hyperconjugation work together to determine its acidity. When the methyl group is present in the *ortho*- and *para*-position both of them can act upon phenol. 

Hyperconjugation as a rule dominates which increases the electron density of the aryl. This makes it less acidic because the conjugate base is now less stable. But when its *meta*-cresol, hyperconjugation no longer works. 

Why is it so? It happens even though an empty $\mathrm{p}$-orbital is present near the $\alpha$-hydrogen. What else does it need?