Which version is correct?
Pieces of gold, silver or black metal, or sometimes all three, are used.
Pieces of gold, silver or black metal, or sometimes all three are used.
So this is actually from a past SAT exam, and the answer key I believe gives the second choice to be correct. I however think it is the first choice, since if we remove the supporting clause "or sometimes all three", the sentence still stands correct as it is.