Heat 1 cup water until steaming but not quite boiling (about 180°F). Pour the gelatin into 8 individual cups and stir together the hot water, sugar, and vanilla extract until completely dissolved. This takes about 2 minutes of constant stirring—don't rush it or you'll have grainy texture.

Divide the clear gelatin mixture evenly between three bowls. Stir the strawberry juice into one bowl, blueberry juice into another, and add the lemon juice to the third bowl for a pale yellow layer.

Pour the strawberry mixture into the bottom of each cup, filling them about one-third full. Refrigerate for exactly 45 minutes until just set but still tacky on top—this helps the next layer stick without sliding around everywhere.

Pull out your cups and carefully pour the lemon layer on top, filling them two-thirds full. Chill for another 45 minutes until semi-set. I know this feels like forever, but the layering is what makes your patriotic jello cups party look professional.

Add the blueberry layer to fill each cup completely. Refrigerate for at least 2 hours until totally firm and jiggly. I usually do this the night before because nobody's got time to wait around on party day.

Top each cup with a generous dollop of whipped topping right before serving. Scatter the diced strawberries, diced blueberries, and mini marshmallows on the whipped topping for that red white blue theme. One time I added these toppings too early and they sank right through the gelatin—trust me, do this step last.

Keep your patriotic jello cups party stored in the fridge until guests arrive. They hold up beautifully for up to 3 days, which makes them perfect for advance prep when you're juggling a million things.