I'm playing in column 3 because it builds upon my existing pieces to create a strong vertical threat. I already have Os in column 3 at rows 4 and 5, so placing another O here will give me 3 vertically connected pieces. This forces my opponent to block the top of column 3 on their next turn, or I'll win by completing 4 in a row vertically. This is my strongest move as it creates an immediate threat while building toward a win condition.