1. The acidic nature of the three nitrophenols increases in the order: meta < ortho < para.
2. Para nitorophenol is the most acidic among the three.
3. The nitro group attached to a conjugated system shows strong -M effect, and this decreases the electron density more at the ortho and para positions than at the meta position.
4. This factor helps greater stabilization of the conjugate base (nitro phenoxide ion) of the ortho and para isomers than that of the meta isomer.
5. So, meta nitrophenol is the weakest in terms of acidity.