Limoncello Collins

In a pitcher, combine the limoncello, gin and lemon juice. Cover and refrigerate until chilled, at least 2 hours. Press 3 thin lemon slices against the inside of each of 8 collins glasses. Add ice to the glasses. Stir the limoncello mixture and pour it into the glasses. Stir 2 ounces of club soda into each drink and garnish with a mint sprig.