If you've done plenty of starters on a stir plate before, I assume you aren't looking for krausen, but rather a milkiness in the starter. The dregs from 3 bottles isn't a bunch of yeast, so be patient. Usually, you would start with a very small amount of wort and use several starters to gradually build to a pitchable count. Give it a day or so and then take a gravity reading to see if it is, or has, fermented.