I guess the norm would be to use "new expressions such as A, B and C" and "a new expression such as A". However, "new expressions such as A" also seems feasible to me. "a new expression such as A, B and C" doesn't sound right, but of course you can use "a new expression such as A, B or C".
New words, one handy idiom, and a 2-minute quiz — delivered to your inbox to keep your streak alive.