According to CME's definition of Open Interest, if Trader A is long 3 contracts, and decides to sell 1, then the open interest decreases by 1.
What I don't understand: if Trader A sells their contract, there must be some Trader B buying it. Then open interest should remain the same. Unless Trader B had a short position, in which case I could agree it decreases by 1. So it would depend on the buyer's position. What did I miss?