**The local bank was robbed by three men**

To convert the sentence "Three men robbed the local bank" into simple past passive, we follow these steps:

1. Identify the subject and object in the active sentence.
- Subject: Three men
- Object: the local bank

2. Move the object to the beginning of the sentence and make it the subject of the passive sentence.
- Object: the local bank

**The local bank**

3. Add the verb "was" or "were" depending on the number of the subject. Since the subject "the local bank" is singular, we use "was" as the helping verb.
- Verb: was

**The local bank was**

4. Use the past participle form of the main verb (rob) to create the passive verb phrase.
- Past participle: robbed

**The local bank was robbed**

5. Add the preposition "by" and the subject from the active sentence (Three men) to complete the passive sentence.
- Preposition: by
- Subject: Three men

**The local bank was robbed by three men.**

In this passive sentence, the focus is on the object (the local bank) that receives the action (robbed) rather than the subject (three men) who performs the action. This construction emphasizes the bank as the affected party.

Passive voice is often used when the subject is unknown, unimportant, or when the focus is on the object. It can also be used to be more formal or to shift the focus of the sentence.

In this case, the active sentence "Three men robbed the local bank" describes the action and emphasizes the perpetrators of the crime. However, in the passive form "The local bank was robbed by three men," the focus is shifted to the bank, which becomes the subject of the sentence, and the emphasis is on the fact that it was robbed.