Before you read any further, open a Bible and read in the book of Genesis 18:1-15. This story is commonly known as the hospitality of Abraham. In this story, three angels appear to Abraham and Sarah. They treat their three visitors with great reverence, and prepare a meal for them. You have probably seen an icon of these three angels seated at the table, but you may have known that this icon is a "type" of the Holy Trinity!
A type is a figure or a representation. The guests are described simply as three men, but when Abraham talks to them, they respond as one ("they said"). The Lord appeared to Abraham, but when he looks to see who is there, he sees three men. Three men speaking as one; the one God in three persons: Father, Son and Holy Spirit. At other times, only one of the angels speaks to Abraham and he is referred to as "Lord."
In the icon, it is most often felt that the angel in the center of the icon is the one referred to as Lord: Jesus Christ. In iconography, it is traditional for Christ to be depicted wearing a red undergarment, with a blue garment over the top of it (red representing His divinity, and blue His humanity which He "put on"). In the icon of the Hospitality, the center angel is also wearing red with blue over the top of it.