If I want to order more than one piece. Do I say "3piece of something" or " 3pieces of something"? I saw that some people use 'pieces' but some people didn't use it. Just used 'piece' without the plural. which one is correct ?
If the context is:
I bought 3 piece(s) of something.
then "piece" is used in its most ordinary form. That is why it should be in plural form, to match "3".
However, there are special uses, where the rule is different:
I bought a 2-piece set
I wear a 3-piece suit.
Here, "piece" is not a stand-alone word, but a fragment of the expression "N-piece (something)".