Thats the most complicated explanation ever.
This is how I solve it. When looking for the conc of Mg3(PO4)2, use this formula N1M1V1=N2M2V2. N=normality. Mg3(PO4)2 has a normality of 3. so
(.005 L)(.005 M MgCl2)= (3)(M2)(.015L) You solve for M2. That give you the concentration of Mg3(PO4)2. Then you just subtract the mole amount of Mg2+ from the concentration found above.
Basically
mole Mg2+ - ((N1M1V2/N2V2)