After upgrade to 3, can I uninstall version 1?

Yes, for sure! Just be aware that MailPoet 3 can’t migrate your past MailPoet 2 newsletters, so if you still want to have access to your past archives and statistics, you’ll need to keep the plugin alive.

Read this later: https://beta.docs.mailpoet.com/article/199-checklist-for-those-migrating-from-mailpoet-version-2