Heat your oven to 350°F (175°C) and grease three 9-inch round cake pans.
In a large mixing bowl, thoroughly combine granulated sugar, all-purpose flour, salt, baking powder, baking soda, and lime-flavored gelatin.
In another bowl, lightly beat the eggs, then add vegetable oil, orange juice, lemon juice, key lime juice, and vanilla extract. Mix until well blended.
Pour the wet ingredients into the dry ingredients and mix until just combined, avoiding overmixing.
Pour the batter evenly into each of the prepared cake pans.
Bake in the preheated oven for 25 to 30 minutes, or until a toothpick inserted into the center of the cakes comes out clean.
Once baked, let the cakes cool in the pans for 5 minutes, then carefully invert them onto wire racks to cool completely before frosting.