H3po4, which contains three hydrogen atoms per molecule, is a weak acid, whereas hcl, which contains only one hydrogen atom per molecule, is a strong acid. explain why?

Because based on atomic structure, in hydrogen chloride chlorine has more
tendency to gain electron of hydrogen and produce H+ and if hydrogens in phosphoric acid give their electrons to oxygens, there will be repulsion among non-joint electrons pair. Also that is why H3PO4 has acid ionization constant  and HCl .
I wish this be helpful for you.

Less stable  hope this

As cooking food that has to go in the oven