This answer is not correct. All of these acids have the acidic hydrogen atom on an oxygen atom. Therefore periodic table effects are not involved in the decision as to which is the strongest acid. The atom to which the hydroxyl group is attached is different in each case. The first step that you should do in order to answer this question would be to draw out the electron dot formulas for each compound and then calculate the formal charge on the atom to which the hydroxyl group is bonded. When this atom has a significant amount of positive charge, it will stabilize the anion formed when the hydroxyl group loses a proton. In the case of nitric acid, the nitrogen atom has a single positive charge which is the same as phosphoric acid but less than sulfuric acid which has a double positive charge on the sulfur atom. Phosphoric acid is stronger than nitric acid because it not only has a single positive charge, it has more oxygen atoms attached to the phosphorus atom which pull electron density from the phosphorus making it more electropositive than the nitrogen atom in nitric acid.