I'm no expert, but I know that when I don't eat anything except Watermelon all morning it really gets my bowels free-flowing! I'll get several loose movements in the a.m.
I've only done 1 entire day on watermelon, not 3. If you have Sugar sensitivity, you could feel weird/jittery/weak/unfocused on all the watermelon. Otherwise I think it's great. Lemon juice is obviously very cleansing, but the Watermelon has the little bit of bulk which probably helps.
Perhaps 1 or 2 days of Watermelon and the (2nd and) 3rd day on lemon juice + water? Plus warm herbal teas.