Given 3 molecules of N, how many moles of NH will be used?

The answer to your question is below

Explanation:

Balanced chemical reaction

                   1 N₂  +  3H₂  ⇒   2NH₃

Process

1.- Consider the coefficients of the balanced reaction (in bold)  

                       1 mol of N₂ 3 moles of H₂

                      3 moles of N₂ x

                        x = ( 3 x 3) / 1

                        x = 9 moles of H₂

2.- Calculate the moles of NH₃

                      1 mol of N₂ 2 moles of NH₃

                       3 moles of N₂ x

                       x = (3 x 2) / 1

                       x = 6 moles of NH₃

yes the answer is total mass in unchanged   : )