I need to block column 3 as my opponent has three X pieces there (rows 2, 4, and 5). If I don't place my piece in column 3, my opponent will place their next piece there and win with a vertical connect four. By playing in column 3, I'm blocking their immediate winning threat while also maintaining presence in the center column, which is strategically valuable. While this doesn't create an immediate winning opportunity for me, defense is the priority in this position.