Fixed tab bar: You should use a fixed tab bar if you expect your users to switch views frequently, where conventional usage entails constant switching between views. And also if you want your users to be highly aware of the different views, like a dial pad on one tab and a contact list on another tab. Views that could be expected to be accessed less frequently, like the settings view, should not be accessed in a top tab structure. The tab bar should conventionally follow with the scroll.
Three is a good number for using a fixed tab bar, just make sure that you phrase it short and concise.
Side navigation (Navigation drawers): Use a side navigation when you want to provide access to top views from a detail view, eg accessing the details of a list item will still enable the user to switch to another perspective without having to go back first to the initial view (which a fixed tab bar would require). And also when you want to provide direct access between views that aren't directly connected to each other. Use this control when you have a large set of views, more than three is a good number to start thinking about it. Also if your app has deep navigational branches.