Many have suggested me to include a sentence like this while asking for recommendations:
"Do you feel that you are able to give me a strong reference?"
I am afraid that my third referee, who was only a professor of one my courses, knows me but not well enough to give a "strong reference". So if I asked him this way, he might ignore my request.
I could also tell him that this is the third letter, so he does not have to exhaustively write about my every strengths. But telling him that he is only the "third” might be considered impolite? Because, emotionally, everyone wants to be regarded as an important person.
What shall I do?