All the three have covalent bonding , within itself. They are also diatomic molecules , that is consisting two atoms in one molecule . Let's start with Chlorine , which has a singly bonded outermost shell to it's opposite chlorine molecule . Oxygen has double bond . Lastly, Nitrogen has a triple bond . Thus, Nitrogen is the strongest cause of it's short bond length , they require more energy to break apart whereas oxygen requires lesser cause of it's double bond and chlorine the least of the bunch.
DO YOU WANT ME TO EXPLAIN IN TERMS OF ELECTRONEGATIVITY OR SUB SHELL THEORY AS WELL ??
