Write balanced equation for proton-transfer reaction between HCl and PO43−?

##i.## ##PO_4^(3-) + HCl(aq) rarr HPO_4^(2-) + Cl^-##
##ii.## ##HPO_4^(2-) + HCl(aq) rarr H_2PO_4^(-) + Cl^-##
##iii.## ##H_2PO_4^(-) + HCl(aq) rarr H_3PO_4(aq) + Cl^-##
So how do you do it?
Any chemical reaction CONSERVES mass AND charge. What do I mean? With respect to conservation of mass, If I start with 10 g of reactants (from all sources), AT MOST I can get 10 g products (and in practice I am not even going to get that). To reiterate, mass is conserved in every chemical reaction. A quantity of sodium phosphate represents a specific mass, and is conserved throughout the reaction.
Not only mass is conserved, but CHARGE is conserved. The 3 reactions reflect this. On the LHS (left hand side) of (i) there was a charge of -3; on the RHS, there is also a charge of -3, as required.
Now it may seem that chemical reactions are quite complicated, but if you look at them, they will always be balanced; that is they conserve mass and charge. It is your job to make chemical sense, and to observe conservation.