Why are There Three Lines for H_B?

HB feels the splitting of both H_A and H_A’. So, let’s imagine starting with H_B as a single line, then let’s “turn on” the coupling from H_A and H_A’ one at a time:

Because the two lines in the middle overlap, that line is twice as big as the lines on the outside. More neighboring protons leads to more lines as shown on the next slide.