The reaction states that for every atom of zinc, there are 2 atoms of hydrochloric acid
since the units are the same, we can simply change "atoms" to "moles"
So: 1 mole Zn + 2 moles HCl works in the equation
Since we have 3 moles of zinc, we can just multiply each part of the chemical reaction by 3 (chemical reactions are kind of like equations).
2 mol Zn + 6 mol HCl --> 3 mol ZnCl2 + 3 mol H2
H2 is hydrogen gas
therefore: 3 mol of hydrogen gas are created