The show bet means that you bet that your horse will finish either on the first, second, or third place. Thus, if your horse finishes first and wins, you also win the bet. However, even if your horse finishes first, you won't get more money as if it would have finished in the second or third place. The amount paid in show bets is unequivocal, meaning that the payout will be the same no matter if your horse finishes first or in the second or third place.