In the bowl of an electric mixer, beat the softened cream cheese, sour cream, ricotta cheese, lime juice, 2 cups sugar substitute, vanilla extract, 1/4 cup hot water, and zest of 1 lime on medium speed until well combined.
Using a fork to mix, thoroughly dissolve the 3 envelopes of gelatin in 3/4 cup boiling water. Do not let cool and move onto the next step immediately. (It is important that the water be boiling hot and that you mix the gelatin powder in quickly so it is completely dissolved. If not, you will have gummy bear-type lumps.) With the mixer on high and working very quickly so the gelatin does not set, blend the hot dissolved gelatin thoroughly into cheesecake mixture. Immediately pour mixture into 10 inch, or deep dish pie pan and sprinkle with the zest of the other lime. Chill in the refrigerator for about 3 to 4 hours or until firm. Serve.
Hint: If you have any extra cheesecake mixture left over after filling the pie pan (I always do), put it in a pastry bag or heavy-duty plastic bag and put it in your freezer for about 7 to 10 minutes, just to firm it up a bit to the thickness of whipped cream. Then take it out and decorate the top of the Key Lime Cheesecake. (If it is in a plastic bag, snip the corner off.).