Key concepts: Stocks -> levels at a point -> time, Flows -> rates over a period -> time.
Stocks are defined -> at a point -> of time.
Flows are defined -> over a period -> of time.
GVA calculation -> formula is correct.
So, correct statements are: (B) Flows are defined -> over a period, (D) Stocks are defined -> at a point, (E) GVA -> formula is correct.
Thus, answer is: 3 -> (B), (D) and (E) only.