The good news is that the least expensive tequila makes the best margaritas, but you must use freshly squeezed lime juice.
Pour the salt onto a small plate. Rub the outside rim of each glass with the cut lime. Dip each glass into the reserved salt; set aside.
Place lime juice, lemon juice, triple sec, and 3 cups ice in a blender, and puree on low speed until completely blended. Add tequila and blend for 2 seconds. Pour margaritas into the reserved glasses, over ice, and serve immediately.