Where to get my third letter of rec?

<p>I'm a junior in Molecular and Cell Biology and will be applying to grad school next fall. Here's where I'm thinking I'll get my letters of rec:</p>

<h1>1 - PI whose lab I will have been working in for 2 1/2 years. Did well in his phage biology class, worked hard in lab, should be fairly strong. I will be doing an honors undergraduate thesis next year.</h1>

<h1>2 - PI from a summer REU program. Should be similarly strong.</h1>

<h1>3 - ??</h1>

<p>I could get #3 from a prof. who I took a couple small "special topics" classes with that were rather open-ended and involved searching and summarizing scientific literature, but I imagine this letter would be rather mediocre. Or, I could get separate letters from my postdoc advisor and PI in lab, but that would be spreading things too thin. Alternatively, I could attempt to do research in a 2nd lab my senior year (I have enough free credit hours) in hopes of getting the PI to write me letter #3. </p>

<p>What are your thoughts?</p>