To subtract fractions, we need to have a common denominator.

For the first subtraction:

8/25 - 4/5

We need to find a common denominator, which is 25.

8/25 - 20/25 = -12/25

For the second subtraction:

-2/7 - 17/42

We need to find a common denominator, which is 42.

-12/42 - 17/42 = -29/42

For the third subtraction:

3/10 - 8/15

We need to find a common denominator, which is 30.

9/30 - 16/30 = -7/30

Therefore, the results of the subtractions are:

1) -12/25
2) -29/42
3) -7/30