Hey there!
3 moles of Cl₂ are needed to react with 3 moles of H₂.
2 atoms of H are reacting with 2 atoms of Cl, the product is made up of 2 atoms of H and 2 atoms of Cl.
So they are reacting in a 1 to 1 ratio, and have the same number of moles.
Hope this helps!
