It does not say if Abraham recognized him immediately or not, we should try not to read our perspective into the text, but at some point he catches on. Abraham was just going about his own normal day at this time he was taking a break because it was hot out. Perhaps he was resting because he was just recently circumcised, all the more reason to be focused on ones self rather then others. When we are tired and ornery it is most likely that we be self focused and miss opportunities to serve others. Notice what the text says, he was obviously not really paying attention to anything in particular for in those days surely you would have noticed three men approaching. "when he saw them" is interesting these men were standing right in front of him but he had not really seen them yet. How often are we like this? how often do we miss opportunities to serve the Lord because we are not looking for them. How often do we miss opportunities when they are right there in our face. I guess we wouldn't know would we, it is a question no-one can answer, how many opportunities have I missed because I have not been looking, I don't know! I would venture to say there are some for all of us. I know there must have had to have been some because I am a sinner and therefore I am sure I have not perfectly kept God's will in this area. When I am hot and tired and on a break the last thing I am thinking is 'how can I serve someone else today' but what is my excuse for when I am not hot, tired, hurt, and ornery and still I do not look and watch and see the opportunities God has given me to be His servant.