Ok, the carboxyl groups, COOH (represented as CO2H below) have the acidic H, So,
R(CO2H)3 -> R(CO2H)2(CO2)- + H+ 1st ionization
R(CO2H)2(CO2)- -> R(CO2H)(CO2)22- + H+ 2nd
R(CO2H)(CO2)22- -> R(CO2)33- + H+ 3rd
Where I used R to represent the rest of the molecule, ie
R is C3H5O