To prepare the simple syrup: In a medium saucepan bring the sugar and water to a simmer. Simmer for 5 minutes until the sugar crystals are dissolved. Cool completely.
Mix the cooled simple syrup together with the pineapple juice, apple juice and lemon juice.
Chill thoroughly for at least 4 hours or overnight.
Just before serving, add the chilled seltzer water, stir well.