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 p orbital is present near the alpha hydrogen.  What more does it need?