You can use one installation to serve three or more single-site setups (just switch on $_SERVER['HTTP_HOST'] in your wp-config.php) or one multi-site installation. But running three updates for the same software on one host doesn’t make sense. Just a waste of time.
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Unless you plan to install hundreds of different themes and plugins on each instance you can share the same plugin and theme directories on multi-site or different single-site-installations.
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The database will not be a problem. And again you can reuse the same data base in multiple single-site installations too.
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There are OpenID plugins to allow such a login. No relation to multi- or single-site.
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There are no big differences. Some plugins work with less problems on single-site installations … and that’s why I recommend a multi-site: You will write your own plugins or themes sooner or later, and writing those for a more complex environment that you are actually using will make you a better developer.