[Jules18]

The lewis diagram of a phosphate group has two -ve signs on the oxygens:

http://student.ccbcmd.edu/~gkaiser/biotutorials/dna/images/phosphate.jpg

So why do text books say the formula is PO₄^-3?
Where is the other -ve coming from?   

[Yggdrasil]

The other singly bonded oxygen should have a -1 charge (note that in reality, the three negative charges on the phosphate ion are distributed equally among the four oxygen atoms because of resonance).