This is a developers forum, so for specifics regarding ads you should visit https://help.twitch.tv
For a brief explanation of why viewers aren’t getting 3 minutes of ads being played though, it depends entirely on how many ads the viewer has watched recently, as well as what ads are available in the viewers region. It may simply be that pool of ads doesn’t have 3 minutes worth of ads that are suitable for that viewer so it played only the ads that were applicable. It’s entirely possible to run an ad break and for there to be no suitable ads available so the viewer just keeps seeing your stream.