coefficient
Let's look at the 3 possibilities and see what they are for 3H₂O₂ coefficient - This is used to indicate that multiple molecules are used for the formula. In 3H₂O₂ that indicates that we are talking about 3 molecules of H₂O₂ subscript - This is a small number set in a smaller font and placed low to the elements. It indicates the number of each type of atom in the compound. For the formula 3H₂O₂ there are 2 subscripts. Both of them being the number "2" set small and low just after the letters H and O. Those subscripts indicate that there are 2 hydrogen and 2 oxygen atoms per molecule.
element - This is the abbreviation for the elements used in the compound. In 3H₂O₂ there are 2 different elements. H to indicate hydrogen, and O to indicate oxygen.
