If I'm using a smaller (3 lb.) brisket, how should I vary the cooking time? Thank you in advance!

I've made it several times with a 3-lb cut and a half recipe, fit snugly in a smaller pot (5.5-quart dutch oven), and the time is about the same, since the meat is about as thick all the way through. You'll just want to watch it a little more closely to make sure it doesn't burn or dry out, adding a bit lof liquid or turning down the heat if it's simmering too rapidly. When it's as tender as you like it's done!