Ortho and para nitrophenols are found to be more acidic than meta nitrophenols . But when it comes to cresols, meta cresols  are found to be more acidic than ortho and para cresols  . Why? 

Remember electron donating groups increase electron density on ortho and para position and electron withdrawing groups they decrease electron density on ortho and para position in aromatic rings
Also remember more electron density decreases the acidity and less electron density increases the acidity.


Nitro groups are electron withdrawing in nature and they can create positive charge on the ortho  and para position which results in increase in acidity as we know that electron withdrawing groups increase the acidity of the compounds.
However in cresol -methyl  group is electron donating in nature which means it is going to be increase electron density  on ortho and para position which results in decrease in acidity for ortho and para but meta position is  not going to acquire the more electron density  so there is more acidic nature if OH is present on meta position.