Physical properties of phenols
In case of p-nitrophenol the lone pair on oxygen of OH takes part in an extended resonance upto NO2 , more delocalisation of lone pair results into more delta minus on H atom ,so stronger H bonding, whereas  in case of m-nitrophenol it cant have resonace structures upto NO2. Since less delocalisation results in comparitively less delta minus, H bonding is comparitively weak.