In a large saucepan, bring 3 cups water, mint and sugar to a boil. Reduce heat; simmer, uncovered, for 10-15 minutes. Stir in the raspberries, lemonade concentrate and remaining water. Refrigerate overnight.
Strain raspberry mixture, reserving liquid. Press berries and mint through a sieve; discard seeds and mint. Combine raspberry juice with the reserved liquid. Serve over ice.