Place corned beef in bottom of instant pot, pour spices over beef. Add 6 cups of water, or enough to cover beef. Secure lid and set Pressure Release to SEALING. Select the MEAT/STEW setting and set cooking time for 60 minutes on high pressure.
Let pressure release naturally after cooking. Don't open the pressure release. Beef will continue to cook and will become even more tender. It will take about 30 minutes to release pressure. Remove beef if ready to eat, you can leave it in to be kept warm for up to 10 hours. Keep 2 cups of the cooking liquid to cook vegetables in. Discard remaining cooking liquid
Before serving, chop up cabbage, carrots, and potatoes into similar sized pieces. About an inch thick. Cabbage can be wedged.
Remove beef from pot. Cover with tin foil to keep warm while vegetables are cooking. Place steamer basket in pot, layer potatoes, then carrots, then cabbage into pot. Secure lid and set pressure release to SEALING. Press CANCEL button to reset the system. Select the STEAM setting, and set the cooking time to 3 minutes on high pressure.
Once vegetables are done, open pressure release to VENTING to quick release pressure. Do not stand directly over Instant Pot.
Carve corned beef into thin slices, and serve with vegetables. Keep cooking liquid to store leftover corned beef in as it will keep the beef moist.
If you like very flavorful beef and vegetables, add another 1/2 teaspoon of pickling spices to beef before it cooks.