Find the missing digits in the subtraction problems.

We have 7 - \text{_} = 3.
So, the missing digit is $7 - 3 = 4$.

We have 4 - \text{_} = 6. Since $4 < 6$, we need to borrow from the Hundreds column. The 4 becomes 14.
So, 14 - \text{_} = 6.
The missing digit is $14 - 6 = 8$.

The 3 in the Hundreds column became 2 after borrowing for the Tens column.
We have $2 - 2 = 0$. This matches the result.

We have $6 - 4 = 2$. This matches the result.

We have 9 - \text{_} = 1.
So, the missing digit is $9 - 1 = 8$.

The completed subtraction for (b) is:

The missing digits are $\boxed{\text{8, 8, 4}}$.

We have \text{_} - 1 = 2.
So, the missing digit is $2 + 1 = 3$.

We have $4 - 8 = 6$. This means we borrowed from the Hundreds column. The 4 became 14.
So, $14 - 8 = 6$. This matches the result.

Let the missing digit be $A$. Since we borrowed 1 from it for the Tens column, it became $A-1$.
We have $(A-1) - 3 = 3$.
So, $A-1 = 3 + 3 = 6$.
Therefore, $A = 6 + 1 = 7$.

We have $5 - 9 = 6$. This means we borrowed from the Ten Thousands column. The 5 became 15.
So, $15 - 9 = 6$. This matches the result.

Let the missing digit be $X$. Since we borrowed 1 from it for the Thousands column, it became $X-1$.
We have $(X-1) - 2 = 4$.
So, $X-1 = 4 + 2 = 6$.
Therefore, $X = 6 + 1 = 7$.

The completed subtraction for (c) is:

The missing digits are $\boxed{\text{7, 7, 3}}$.