Fill a large stockpot with 3 gallons of water and bring it to a boil over high heat.
Add the kosher salt, Old Bay seasoning, crab boil seasoning, and lemons to the pot. Stir to combine.
Add the onions and garlic to the pot and continue boiling for 10 minutes.
Add the red potatoes to the pot and boil for 10 minutes.
Add the corn and sausage to the pot and boil for an additional 10 minutes.
Add the live blue crabs to the pot and boil for 10-15 minutes or until the crabs turn a bright orange color.
Turn off the heat and let the crab boil sit in the pot for 5-10 minutes to allow the flavors to meld.
Using a slotted spoon, transfer the potatoes, corn, sausage, and crabs to a large serving platter.
Serve the crab boil with melted butter and lemon wedges on the side.
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