PO3^-3 is also called a phosphite ion. In order to draw its electron dot structure first we need to get the total number of valence electrons.
P = 5
O = 6 x 3
e = 3 (negative charges add to the number of electrons)
total of 26 electrons
Next we connect the atoms with P being the central atom.
O - P - O
And then we supply the remaining electrons around the atoms.
.. .. ..
: O - P - O :
'' | ''
: O :
Double check the electrons assigned if it is similar with the total number of electrons counted earlier. (One bond corresponds to 2 electrons.)
The electron geometry is tetrahedral and the molecular geometry is a trigonal pyramidal.