I only have 2 research experiences, one of which lasted more than a year and is starting to look very promising for high impact factor publication (I'll be the second author though, and it's not likely to be published by the time of application). The other is with an highly esteemed professor from the U.S. who's quite happy with my 2-month research internship in his lab. Both are willing to put in some good words for me, but that's all I got, so I need a "did well in class" letter of recommendation which will probably be worth nothing. Will this hurt a lot to my graduate application? (I'm an international student)
All of your fellow students are in the same boat: almost nobody has more than 2 successful research experiences (and many have fewer). You don't have to have a stellar file when applying -- it just needs to be as good or better than that of all of the other candidate, most of whom will mostly have "did well in class" letters.
In other words: you're ahead of the curve. Nothing to worry about.