Answer: had been working.
Explanation: In English, the past perfect progressive is a verb tense which is used to show that an action started in the past and continued up to another point in the past. It is made up of the past perfect form of the verb "to be" (had been) plus a present participle (ending in "-ing"). So the past perfect progressive of the verb "work" is "had been working" and the complete sentence would be: Trisha had been working at the video store for only three months before she got a raise.
