Pour the sugar and 1 cup water into a small pot and place over medium heat. Bring to a simmer, stirring until the sugar is dissolved. Remove from the heat and add the mint sprigs. Let the mint steep for 10 minutes, then discard the mint.
Add the mint simple syrup to a large pitcher, punch bowl or serving canister. Add the lemon juice, blackberries, raspberries, strawberries and lemon wheels and give a good stir. When ready to serve, fill the pitcher halfway with ice. Top the mixture with seltzer and give a good stir. Serve over ice with sprigs of fresh mint.