The odds against winning is a ratio of 3:1, meaning the odds against winning are 3 to 1. Thus, the probability of winning is 0.25, or 25%.

3 & 1: This means that one player had a two-shot lead going into the 17th hole, and won that hole  so they won by three and there was just a single hole to play. They played 17 because had their opponent won the last two holes, the match could have ended in a draw.