What is the degree of unsaturation of oxalate anion?
The Degree of Unsaturation can be considered as the number of Pairs of Hydrogen Atoms that could be added to the structure. One can simplify the structure to consist of two carbon-oxygen double bonds and two carbon-oxygen single bonds in one resonance contributor. Note that the single bonded oxygens have a negative charge and thus could accept a hydrogen each. This accounts for 1 degree of unsaturation. There are also two double bonds, and a pair of hydrogens could be added across these also. This gives two degrees of unsaturation. Hence there are 3 degrees of unsaturation in total (as the book says).

In order to calculate the degree of unsaturation for any compound, one can use the formula:
((2* Number of Carbons) + 2 - (Number of Hydrogens and halides) + (number of nitrogens)) / 2.
In this case, the formula gives 3, which is the degree of unsaturation.