Since the areas under the three curves are proportional to the relative size of the populations, it means if the three populations are combined, and taking their sizes into account, the combined mean will be the same as the mean of the whole population. It's the same as if you had a teeter-totter with a very large kid sitting one foot to the left of the pivot point, a small kid sitting about seven feet to the right, and a baby sitting about thirteen feet to the right.
Now here is a graph from 1990. Note that the mean IQs of the three populations have shifted: the non-college mean has dropped to about -0.25 SD below the overall mean IQ; the college grad mean has gone up to about +0.85 SD above the overall mean IQ, and the mean IQ of the grads of the prestigious colleges has taken a big jump to about +2.8 SD above the overall mean IQ. Quite a change in sixty years. As the authors predicted when they wrote the book in the mid-90s, this shift has profound social consequences.
At least one other author I read recently has confirmed, in a way, that what Herrnstein and Murray predicted has happened.