The third one
4/8 + 3/8 = 7/8
11/8 - 3/8 = 8/8. If you have 8/8 of a pizza, you still have the whole (1) pizza. Any number divided by itself is 1. 8/8 = 1
1 1/8 + 3/8 = 1 4/8, not 4 1/8, when you add a fraction to a mixed number, you still add the numerators to each other, not the number in front. If you need to draw it out, definitely draw it out to see why you add the numerators.
1 6/8 - 3/8 = 1 3/8. Just like above when you subtract a fraction to a mixed number, you subtract the numerators from each other.