Genesis 18

It is interesting to go back and read some of the old stories that I have known the gist of for so long. I didn't recall the first half of this chapter, which portrays the Lord as being amongst three men on their way to Sodom and Gomorrah. I can't tell whether or not the men know that the Lord is with them. Is He there as a man? Or does Abraham recognize the spirit of the Lord as being there? Or is He one of/all of the three?