Mostly, yes. Snapchat is designed in a way that friends can view each other's stories. But keep in mind that the person posting the story has control over privacy settings. For example, if someone goes into their settings and selects 'Custom' for story viewing, they can pick and choose which friends can see their story. So while the general idea is that friends can see each other's stories, it's not always the case depending on individual privacy choices.
Yes, typically, Snap Chat friends can see each other's stories if they have the proper settings enabled and are allowed by the person sharing the story.
Not necessarily. While Snapchat is built around the concept of friends sharing and viewing stories, users have the ability to customize who sees what. Some stories might be set to private, or only viewable by a specific subset of friends. So it's not automatic that all friends will see every single story that another friend posts.