I want to continue building on the column where I already have an O. By dropping my piece in column 3, it will fall on top of my existing piece at row4, creating a vertical stack of two O’s (positions at row4 and row3). This not only develops a potential for a vertical connect-four but also creates more chances for diagonal connections in the future. At the same time, I don't give any immediate advantage to my opponent. Therefore, I choose column 3.