Tear the mint leaves into pieces and put 1/2 cup into a cocktail shaker with 1/4 cup lime juice, 1 tablespoon sugar, and 4 ounces tequila. Add crushed ice and shake well.
Strain over ice into high ball glasses and top off with soda water. Garnish with mint leaves and lime slices.
Empty the shaker and repeat the steps to make the remaining 2 drinks.
Notes
If you don't have a jumbo-sized cocktail shaker that exceeds 24-ounce capacity, make the drinks in two batches. Rinse out the shaker in between drinks.