Sa'id bin al-Musayyib (may Allah have mercy on him) said: There is no harm in wagering on a horse race when they bring in a third horse as well. If it wins, it takes the stake; and if it loses, nothing is due from it.
Hadith Referenceالسنن الكبرى للبيهقي / كتاب السبق والرمي / 19772