How much does C3 weigh?

If you mean 3 atoms of carbon, you would multiply the atomic weight of carbon, 12.011 u/atom (on the Periodic Table) by 3.

3 atoms C x 12.011 u/atom = 36.033 u C

If you mean 3 moles of carbon, you would multiply the molar mass of carbon, 12.011g/mol (atomic weight in g/mol) by 3.

3 mol C x 12.011g/mol =36.033 g C