Fill a highball glass with ice cubes. Add grape juice, orange juice and lemon juice. Stir. Top up with soda water.
A good host or hostess thinks of everyone - don't be one of those people scrounging around in the back of the fridge for a sole diet soda. Have a plan for the non-alcholic drinks before the guests arrive. If you're prepared for mixed drinks, you've likely got a good base for making nice mocktails: juices, soft drinks, ice and fresh fruit will take you far. And experimenting is so much cheaper than with alcoholic drinks!