Prep your workspace with a wooden cutting board, large chef's knife, large mouth, air tight, mason jar or fermenting jar.
Rinse the cabbage in the sink to get all the dirt off.
Peel the outer layers of the 3 cabbage heads off. Set a couple large leaves aside to use on top of the sauerkraut later.
Slice all 3 cabbage heads in half. Then take each half and slice in half again to make quartered wedges. This will leave you with 12 wedges.
Core each wedge by standing the wedge up on the cutting board slicing downward from where the core beings (about 1/4 way down the center of the wedge). Toss the cores in compost (or garbage).
Shred the cabbage with the chef's knife by lying the wedge down on one of the flat sides, then slicing from the tip of the wedge down to the butt in 1/4 inch shreds.
Pile the cabbage shreds into your largest glass or enameled bowl (not metal).
Pour the salt onto your cabbage shreds.
Start massaging the cabbage using clean hands. massage for about 15 minutes, or until the shreds shrink, and create a brine.
Pack the cabbage shreds into your jar of choice making sure to pack tightly so the brine comes to the surface overtop the cabbage. Leave several inches at the top for your fermenting weight.
Cover the cabbage with the outer cabbage leaves you saved earlier making sure the brine comes up over top the leaves.
Add your fermenting weights (or rocks inside plastic) to the top of the cabbage leaves.
Close the lid to the jar.
Contain your jars into a bigger dish to catch any condensation.
Wait for the cabbage to ferment for several days. Taste after 3 days to see if it's sour enough for you. If not, let ferment longer.
Store in the fridge. Sauerkraut is fully mature after 6 months but much of what you'll read says it goes bad shortly after that.