This is a matter of pattern recognition.  If you look at the first one you notice that of all the possible faces showing only the three and four are not present.  This is a sum of seven.  Let's look at the next one:  two, three and four are not showing giving a total of nine.  In other words, the answer is the sum of the faces not showing from all of the die.  The third one has only one's showing, so the faces not present are 2, 3, 4, 5 and 6 giving a total of 20.

So, the answer for the three at the bottom are 15, 12 and 9.