Increasing Acidic Order:
4–Methylbenzoic acid has a pKa of 4.362
3–Methylbenzoic acid has a pKa of 4.269
Benzoic acid (pKa=4.19).
2–Methylbenzoic acid has a pKa of 3.90
The methyl group at ortho & para-position(4-position) destabilises Negative charge of corresponding Carboxylate ion both +I (Inductive ERG)) as well as by +H (hyperconjugation).
The +H effect by methyl at meta position does not affect the carboxylate.(From here only +I effect operates)
Hence m-isomer is more acidic than para-isomer.
2-Methyl Benzoic acid (or o-toluic acid) is stronger than either because steric hindrance results in the carboxyl group being out of the plane of the aromatic ring. [Ortho effect- o-toluic acid is even stronger than benzoic acid.] The carboxy group being out of the plane in o-toluic acid eliminates the factor of +H & increase acidic character by also inhibiting +R effect of benzene (which somewhat destabilises negative charge of carboxylate ion by cross-conjugation).