Dic: three cheers (for someone or something)

▷ three cheers : three shouts of hurrah given in unison by a group to honour someone or celebrate something

▷ three cheers for sb! spoken. used to tell a group of people to shout three times as a way of showing support, happiness, thanks etc

• Three cheers for the birthday girl!

.... Collins, LDOCE

CF. two cheers for something, not three.