From whom do prospective grad students get recommendation letters?
In general, I don't think all that many students have three unique reference letters, all of whom can vouch for research ability. Two is quite common, as most undergraduates who pursue graduate degrees (at least in the sciences) do have some research experience at their home universities. Many of them also pursue a summer research project separate from their main undergraduate research project (or projects), providing another reference.

But I do agree that the third letter usually comes from a non-research source. I wouldn't even necessarily expect a third research letter.