1) 2 - Since they ONLY like hamburgers, you should look at the part where it is ONLY hamburger - no combinations
2) 18 - If you see the word "or", it means both of the circle. Therefore, you have to add up all the numbers inside the "pizza" circle AND the "taco" circle. It does not matter if any pizza or taco lovers also like hamburgers because they are asking who likes pizza or taco (and it means EVERYONE who liked pizza and EVERYONE who liked taco).
3) 10 - Once again, if you see the word "or", it means THE WHOLE THING. Therefore, you should look at the number that is not in any of the circles.