A 3lb roast yields approximately 8 cups of shredded beef. Honestly more than it sounds like.
Slice the roast into 4-6 chunks and sear in a hot skillet, until all sides of each piece are browned.
While the meat is cooking, add all other ingredients to the slow cooker, and give them a stir to combine. They don't need to be completely mixed. Top with the cooked meat and set the slow cooker to low for 8 hours.
If you are leaving the slow cooker for the whole eight hours, pull some of the seasonings and vegetables from the bottom of the slow cooker on top of the beef, so that it's surrounded by the other flavors. If you'll be able to check the beef periodically, give it a stir every hour or so.
After the cooking time is up, shred the beef right in the slow cooker with two forks - it should fall apart very easily.
If using for tacos, drain some of the liquid off the meat before serving.