Sentence structure + impersonal construction

[abc71044]

Excuse me. I want to know what is the error in the following sentence and what are the rule on that.

1. There have many reasons why Hong Kong people are considering job opportunities on the Mainland.

2. It is no doubt that shortage of clean water is a problem many countries will face in the future

3. By the time the fire was extinguished, there were five fire trucks arrived at the scene.

[Tom]

[abc71044]

Thank you Tom.
But can you briefly explain that why should do these change?
Thanks