It is debatable as to why God appeared in 3 men to Abraham in Genesis 18. Tradition, especially Eastern Orthodox tradition, holds that this visitation was representative of the trinity. But there has been debate over the subject since the earliest Christian theology. Augustine believed that God’s appearance here was representative of the trinity, siting that there is no hierarchy. However, others note that when the visitors go to Sodom to warm Lot, only two go and they are called angels.
No matter where you come down on the issue though it doesn’t not take away from the important part of the text which is that God met with Abraham again and confirmed his promise with him.