Can I use WP to create a multisite importing the posts of 3 old WP website?

Yes, you can import content from other single site installations into one multisite installation (network). Widgets, themes and plugins are set per site in a network. What you cannot do is using multiple versions of the same theme or plugin, eg. version 1.2 on one site and 1.3 on another.

Before you do that, set up a local test installation. See Multisite on Windows with wildcard subdomains or WordPress in a nutshell: WT-NMP for examples.

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