Use a large crockpot that can accommodate the roast plus all the vegetables you want to add. Consider cooking the vegetables separately if there isn’t enough room to fill the slow cooker with the liquid.
Rinse the corned beef and place it in the crock pot. Surround it with vegetables. Add the spice packet or the spices listed. Pour in the beef broth and as much of the three cups of water as you need to cover the meat entirely.
Set the slow cooker on medium heat. Begin checking the corned beef for doneness around 5 to 6 hours after you start cooking. Aim for a texture where a fork easily pierces the meat to the center, but it’s not falling apart at the slightest pressure yet. Keep cooking if the corned beef still seems too firm.