I picked up a pregnant stray 3 weeks ago, and she now has diarrhea, is she due to deliver any day now?

Annie, diarrhea isn't necessarily a sign she is about to give birth. It could be caused by any number of things. Since we cannot examine her online, it is impossible to tell how far along she is. Please contact a local vet and have her examined. Make sure her health is good and the vet can give you directions on how to help her and what to do once the kittens are delivered.