An acid with molecular formula ${\mathrm{C}}_7{\mathrm{H}}_6{\mathrm{O}}_3$ forms only three types of sodium salts, i.e. ${\mathrm{C}}_7{\mathrm{H}}_5{\mathrm{O}}_3\mathrm{Na} , {\mathrm{C}}_7{\mathrm{H}}_4{\mathrm{O}}_3{\mathrm{Na}}_2$and ${\mathrm{C}}_7{\mathrm{H}}_3{\mathrm{O}}_3{\mathrm{Na}}_3\text{. }$The basicity of the acid: 

The quantity of hydrogen ions that can be ionised in one acid molecule is known as the acid's basicity.
Three swappable H atoms are shown in this molecule. So, its fundamentality is 3.