Final answer:
The subscript for Hydrogen in the chemical formula ЗН₃РО₄ is 3, indicating there are 3 hydrogen atoms in a molecule of phosphoric acid.
Step-by-step explanation:
The subscript in a chemical formula represents the number of atoms of the element that are present in a molecule. In the chemical formula ЗН₃РО₄, the subscript for Hydrogen (H) is 3. Therefore, there are 3 hydrogen atoms in a molecule of phosphoric acid, which is represented by this chemical formula. When considering multiple elements in a molecular formula, each atom's subscript is written directly after the element symbol, indicating how many of those atoms are present. It's important to not confuse subscripts with coefficients in front of a formula, which indicate the number of molecules.