Fact family refers to the sum of addends of addition, as well as subtraction, minuend and the difference converted from it.

For example, in your chart, the top is 3, and the bottom is 1 and 2. These three numbers constitute the fact family. Subtraction from top to bottom and addition from bottom to top. The operations involved are:

1 + 2 = 3, 2+ 1 = 3

3 - 1 = 2, 3 - 2 = 1