Pop the three bags of popcorn and pour them into your very large pan.
Preheat the oven to 200°.
Bring the brown sugar, butter, dark Karo syrup, and Cream of Tartar to a soft boil over medium heat in a medium sized saucepan.
Remove from the heat and mix in 1 tsp. baking soda. Stir until frothy.
Pour this caramel mixture over your popped popcorn that is in your large oven safe pan.
Stir.
Bake for 2 hours at 200°.
Stir every 30 minute or so.
Cool completely and then put in airtight containers/container.