Is this a joke, or an actual experience? I could kind of believe that if they were unfamiliar with the products they might confuse them, but 7-Up has a distinct lemon-lime flavor, and Fanta is, well, a WWII poverty drink.
An actual experience. There were many more wrong guesses than you would think, which isn't that surprising given the insane quantity of sugar in all three of them. Your eyes do a lot of work when tasting food and drink, so when you see yellow bubbly sugar-water you assign a much poorer experience to it than the brownish bubbly sugar-water.
I did a blind test of 6 brands of cola with a friend who claimed you couldn't taste the difference, and got them all right. In fairness, 3 of them were with artificial sweeteners, which is very easy to identify.