Place six 8-ounce canning jars on a rack in a water bath canner or stockpot.
Fill the jars and canner with water to cover jars by at least one inch. Cover the canner or stockpot and bring to a simmer over medium heat. (Do not boil.)
Place six canning lids in a small saucepan and heat over low heat. Do not boil. Remove from heat and set aside but keep lids warm. Wash and dry canning rings. Do not heat in a saucepan. Set aside and keep clean.
Place the 3 cups of sugar in a bowl and set aside.
Wash strawberries in a colander and drain well. Hull strawberries. Working in batches, place strawberries into a pie plate and mash with the back of a fork or a potato masher. Measure out a total of 5 cups of mashed strawberries and add to a deep-sided pot that is either stainless steel or enameled cast iron.
Place pot with mashed strawberries on the stove. Add the lemon juice and the pectin to the strawberries and mix well with a whisk until all the pectin is dissolved.
Turn on the stove burner and bring the mashed strawberry mixture to a rolling boil over high heat. Stir continually. Once you have a full rolling boiling that you can't stir down, add in all the sugar at once. Continue to stir and keep at a high boil for 1 minute.
Remove from heat and skim off any foam that has risen to the top.
Using a jar lifter, remove one jar from the canner and empty the water from the jar pouring it back into the canner. Place jar onto a counter that is lined with a thick dish towel. Do not dry the jar. Using your index finger, run a clean paper towel dipped in vinegar around the top of the jar to clean it and make sure there are no nicks. Immediately fill the jar with the hot jam using a funnel and a ladle. Leave a 1/4 inch headspace.
Use a de-bubbler inserted into the jar to make sure there are no air pockets. Top off jar with a bit of extra jam if it goes below the 1/4 inch headspace. Clean the top of the jars one more time with a paper towel dipped in vinegar.
Use a lid lifter to lift jar lid out of saucepan filled with warm water. Place lid onto jar and add canning ring. Tighten ring until you meet resistance and then apply one more turn to "fingertip" tight. (See video.)
Continue this process until all the jars are filled and placed into the water bath canner.
Once all the jars are in the canner, put the lid on and bring the water up to a boil. Once the water boils, set a timer for 10 minutes. Allow the jars to boil in the water bath canner for 10 minutes. After 10 minutes, turn off the heat, and carefully remove the lid. Allow the water to stop boiling and the jars to rest. (About 5 minutes.)
Use the jar lifter to remove the jars from the water bath canner. Keep them straight. And place them on the towel-lined counter. Check them in 24 hours to make sure that the seals are tight with the lids slightly concave and not able to be moved or removed by hand. Remove rings. Jam is now shelf-stable for at least 12 months or up to 18 months, depending on the canning lids used.