Here are the completed sentences with explanations:

3. Sharon eventually **found** the job she **had wanted** last year, although she **had graduated** the year before.
- *"had wanted"* shows that her desire existed before she found the job.

4. When the doctor **arrived** at the scene of the accident, he **realized** the victim **was still breathing**.
- *"was still breathing"* indicates the condition of the victim at the time of arrival.

5. The workmen **had spent** all morning digging the trench, and by lunchtime they **finished** it.
- *"had spent"* indicates the action that was completed before another action (finishing the trench).

6. When the ship **hit** the reef, most of the crew **were playing** cards, and the passengers **were having** dinner.
- *"were playing"* and *"were having"* indicate ongoing actions at the time the ship hit the reef.

7. When she **was** a child, she always **wanted** to do the most dangerous things.
- *"was"* indicates her state at that time, and *"wanted"* expresses her repeated desire.

8. I suddenly **recognized** the town square. I **had been** there ten years before.
- *"had been"* indicates a past state before the recognition occurred.

9. He **gave** me advice after I **had made** the decision.
- *"had made"* indicates the decision was completed before he gave advice.

10. On my last visit to my hometown, I **found** that it **had changed**.
- *"had changed"* indicates a change that occurred before the visit.

### Explanations:
- **Past Simple** is used for actions completed in the past.
- **Past Perfect** indicates an action that was completed before another past action.
- **Past Continuous** describes actions that were ongoing at a specific moment in the past.