Bring 3 cups water to a boil in a medium saucepan over high heat. Lower the heat to a simmer. Have ready a medium heatproof bowl that can fit over the saucepan without touching the water.

Combine the unflavored gelatin with 1/2 cup cold water in the bowl and stir frequently until all the gelatin is moistened and becomes a thick paste, about 5 minutes.

Lightly grease a 9-by-13-inch baking dish with nonstick cooking spray. Set aside.

Place the bowl of bloomed gelatin over the simmering water to gently melt, stirring occasionally, until no lumps remain, about 30 seconds. Remove from the simmering water and stir in 1/2 cup plus 2 tablespoons of the condensed milk until smooth. If there are lumps, place the bowl over the simmering water and stir occasionally until smooth. Stir in 1/2 cup of the vodka, then pour into the prepared baking dish and refrigerate until just set and sticky when touched, about 30 minutes.

Whisk the orange-flavored gelatin with 1 cup of the boiling water in another medium heatproof bowl until well dissolved, about 2 minutes. Whisk in 1/2 cup cold water, 1/2 cup of the remaining vodka and 1 tablespoon of the condensed milk until well combined. Skim the bubbles from the surface with a spoon (see Cook’s Note for an alternative method). Let cool at room temperature until the white layer in the baking dish is set (see Cook’s Note). Pour the orange gelatin mixture over the white layer and refrigerate until just set and sticky when touched, about 30 minutes.

Whisk the lemon-flavored gelatin with the remaining 1 cup boiling water in a third medium heatproof bowl until well dissolved, about 2 minutes. Whisk in 1/2 cup cold water, the remaining 1/2 cup vodka and 1 tablespoon condensed milk until well combined. Skim the bubbles from the surface with a spoon. Let cool at room temperature until the orange layer in the baking dish is set. Pour the lemon gelatin mixture over the orange layer and refrigerate until firmly set, at least 90 minutes, up to overnight (see Cook’s Note).

Lightly grease a plastic cutting board or baking sheet with nonstick cooking spray. Run a thin knife or offset spatula around the outside of the layered gelatin and invert onto the prepared cutting board. Trim the edges for neat corners and cut the gelatin into 40 equal pieces. Transfer to a serving platter.

Whip the heavy cream and sugar with a handheld mixer or whisk in a large bowl until firm peaks form. Transfer to a piping bag fitted with a large star tip. Close the top of the bag and refrigerate until ready to serve. Pipe a small star on top of each Jell-O shot and sprinkle with the sprinkles. Serve immediately.