I am applying next cycle so just under a year away. I'm currently doing a large quantity of biophysics research with a professor and writing a thesis on the research. I am very confident in a high quality LOR from this professor. I've done research in the past with my adviser as well as taken classes with him and expect a strong letter from him also. Here's where my question comes in: for my third letter I have a professor who knows my intellectual capabilities well and would support those. However, since a PhD is a research degree, would it benefit me in a significant fashion to add a 5-10 hour a week project with an additional professor for the next year so that all three LOR writers can discuss research I've done? Does it not matter as much because it's my third letter?