Explanation:

- Option 1: was

- "Was" is used for past continuous tense but does not fit well with "for the past three years."

- It suggests a single occurrence, not ongoing action.

- Option 2: have

- "Have" is the present perfect tense, indicating action started in the past but doesn’t emphasize continuity.

- Lacks emphasis on the action being ongoing over time.

- ?? Option 3: have been

- "Have been" is the present perfect continuous tense.

- It indicates an action that started in the past and is still ongoing, fitting perfectly with "for the past three years."

- Option 4: had been

- "Had been" is past perfect continuous tense.

- It implies an action was ongoing before another occurred but doesn’t match "for the past three years" in the present context.