Hi I have a question on oxidation.When doing the oxidation number for certain questions, how do you figure out which element you are solving for? I already know that if there are only 2 elements (Oxygen and something else), you solve for the other element because Oxygen will be -2 most of the time. But let's say you have questions like these:

NiH^3

PCl^5

H^3PO^3

Which element do you solve for?