나는 밥먹을때 책을 자주 봤(었)다.가 좀 더 자연스러운것 같아요. 사실 언제쓰는지는... 설명하기가 어려워요 ㅜㅜ 선생님께 다시 물어보세요.

Do you want to say 나는 자주 밥을 먹으면서 책을 읽어요? I often have meals reading a book.

감사드립니다! You were so clear, thank you kindly!

1. 나는 자주, 밥먹을 때 책을 읽었어요. 2. 나는 밥먹을 때 자주 책을 읽었어요. 3. 나는 밥먹을 때 책을 자주 읽었어요. These three would be good enough to use. First, 먹었을 sounds like the event happened only one time. I know where your are coming from. Probably because the western people normally say "When I was eating, When I'm eating, When I will be eating" revealing tenses clearly. I can't come up with the exact grammar rule right now, but it's totally fine to use "먹을 때" for all the tenses in Korean. Even though it sounds to you as if "When I'm eating". Just consider that like "When eating", then it'd be easy for you to understand, I might add. Secondly, you should be careful where to place 자주. 자주 which means 'often' should be connected to the main verb. So place it close to '읽었어요' like 2 and 3, or put a comma to show that the word is not related with 밥먹을, the verb in the dependent clause, like 1.