Erin,
I don't think that there is any explicit guidance on this, but I would be hesitant to turn in an application that has three essentially duplicate entrires. I would choose to highlight different items on the different projects.
However, your statement that you've had "the exact same work experience each year" and that you've had "years of doing the same 3 projects" makes me wonder a bit -- by PMI's definition, a project is a temporary endeavour. Surely if these are indeed projects, they must be moving through their lifecycles, and each would have presented unique challenges and experiences over the course of the years that you've been working on them?