i need help with past perfect tenses!

Hi, I wonder whether you can help me with the past perfect. I really need help. So, ok. If there are more than two verbs in a one sentence and each one of them happend before a nother, where should I put the past perfect form???? example(in my opinion, I put the past perfect tense in the verb which happend first): Tom was sad(3), because somebody broke(2) into his house, which he HAD LEFT(1) unlocked.

[Tdol]

That's correct, but you could also put 'had broken' as both actions were happened before he was sad.