Start by preparing the glassware. Using two small plates, pour the margarita salt onto one plate and simple syrup on the other plate. Take your glassware and dip the rims in the simple syrup and then the margarita sanding sugar and set to the side. Then, fill your glasses with ice 3/4 of the way full.
Remove the stems from the raspberries and blueberries. Hull the strawberries. Combine the berries in the blender with the lemon and lime juice. Puree for 10 seconds.
To the blender, add 3-4 pieces of ice, tequila, triple sec, and sweet and sour mix. Pulse to combine.
Pour the mixture into the glasses, and top each off with a Cointreau float. Garnish with a lime wheel.