Ok, here are the pKa2 values for the hydroxybenzoic acids:
o-hydroxybenzoic acid (salicylic acid): 13.4
m-hydroxybenzoic acid: 9.92
p-hydroxybenzoic acid: 9.32
The hydroxyl group on salicylic acid can form hydrogen bonds with the carboxylate group on the same molecule. Because of this, the hydrogen doesn't leave so easily and its pKa is much higher.