If you eat just apples and water for 3 days how much weight will you lose?

Too many variables. How much salad? What is the starting weight? What is the activity level? Is there enough food to prevent the body going into starvation mode? Such a limited diet is likely to be low in protein which can cause health problems. Vitamin levels can go bad. And for a female the lack of good iron sources without meat can be very unhealthy.