Arrange 24 two-ounce plastic shot cups on a tray or baking sheet for easy transfer.
For the purple layer, whisk grape Jell-O with 1 cup boiling water until dissolved, then stir in ½ cup cold vodka and ½ cup cold water.
Pour about 1 ⅓ tablespoons into each cup and refrigerate until set, about 45–60 minutes.
For the orange layer, whisk orange Jell-O with 1 cup boiling water until dissolved, then stir in ½ cup cold vodka and ½ cup cold water.
Carefully pour about 1 ⅓ tablespoons on top of the purple layer in each cup, then chill until set, about 45–60 minutes.
For the green layer, whisk lime Jell-O with 1 cup boiling water until dissolved, then stir in ½ cup cold vodka and ½ cup cold water.
Gently pour about 1 ⅓ tablespoons over the orange layer in each cup, then refrigerate until fully set, about 1–2 hours.
Just before serving, top each shot with whipped cream and Halloween sprinkles.