I am now looking for people to write recommendation letters for me so as to prepare for graduate school application by the end of this year. I found I still need one more person(I want to have 3 letters, as is required by almost all US graduate schools), but I don't know what does it take to get a recommendation letter that is worthwhile? I mean, is it OK to ask a professor write a recommendation letter for you only based on your behavior in his/her class? or you must do research with a professor for some time so that the letter can contain something really meaningful? (The problem I have with the second choice is that I am already participating research with a professor, and I guess it is not a good idea to distract myself to another professor's field at this time.)
I mean, is it OK to ask a professor write a recommendation letter for you only based on your behavior in his/her class?
This is fine for one letter, especially if you did really well in their class, went to office hours, got to know them, etc. As you say, your main letters should come from research advisors or people who know you better than just one class.