Each dog eats 1/2 x 2 = <<1/2*2=1>>1 pound of dog food each day.
So the three dogs eat a total of 1 x 3 = <<1*3=3>>3 pounds each day.
That is 3 x 7 = <<3*7=21>>21 pounds in a week.
Therefore, 30 - 21 = <<30-21=9>>9 pounds of food are left after a week.
#### 9