Simmer 3 pint-size jars in a large pot of water until jelly is ready. Wash the lids and rings in warm, soapy water.

Slice ends off each prickly pear. Make 1 long vertical slit down each one. Use slit to hold skin and peel off. Discard peel.

Place peeled pears in a blender; purée in batches until liquefied. Press puréed mixture through a fine-mesh sieve set over a bowl. Discard pulp and seeds.

Measure out 3 cups prickly pear juice and pour into a large pot. Add lemon juice and pectin; bring to a boil, stirring constantly. Add sugar and return to a boil, stirring constantly. Let cook at a boil until thickened, about 3 minutes. Remove from heat.

Pour prickly pear jelly into the hot, sterilized jars, filling to within 1/4 inch of the top. Run a clean knife or thin spatula around the insides of the jars to remove any air bubbles. Wipe the rims of the jars with a moist paper towel to remove any food residue. Top with the lids and screw on the rings.

Place a rack in the bottom of a stockpot and fill halfway with water; bring to a boil. Lower jars 2 inches apart into boiling water using a holder. Pour in more boiling water to cover jars by at least 1 inch. Bring to a rolling boil, cover the stockpot, and process for 15 minutes.