The second dog eats 1.5 x 2 = <<1.5*2=3>>3 cups of dog food.
The third dog eats 3 + 2.5 = <<3+2.5=5.5>>5.5 cups.
So, Hannah should prepare 1.5 + 3 + 5.5 = <<1.5+3+5.5=10>>10 cups.
#### 10