To a large pitcher, add the strawberries, cucumbers, mint and lemon, lime and orange slices. Pour in the prosecco, Pimm's, Simple Syrup and lemon juice and stir to combine. Let stand at least 30 minutes for the fruit to impart its flavor to the drink.
Stir in some ice to chill down the cocktail, then serve in glasses over ice, adding some fruit and cucumber to each glass.
Combine the sugar and 1 cup water in a small saucepan and bring to a boil. Cook, stirring, until the sugar dissolves. Remove from the heat and let cool completely. Keep covered in the refrigerator up to 1 month.