The important thing to remember is that the posts are hollow and on their own will not support either fence or gate. Gate posts need to be more substantial than other posts. If the concrete goes in the hole first concrete can be worked up into the post somewhat but this is not reliable so I would eliminate No1. A pressure treated wood insert (no. 2) works well but there needs to be gravel in the bottom of the hole as drainage to prevent rotting of the wood. I am not familiar with metal inserts but this could be more durable ask to see a demo: the strength needs to come from the insert and the vinyl just like a sleeve over it.

See this thread:

Here is a link that might be useful: Vinyl fence posts - cement or not?