setbeamertemplate{itemize item}[]

Main text. Main text. Main text...
   * Item 1
   * Item 2
   * Item 3
   ....

Main text. Main text. Main text...
* Item 1
* Item 2
* Item 3
....

setlength{leftmargini}{0pt}

makeatletter
renewcommand{itemize}[1][]{%
  beamer@ifempty{#1}{}{defbeamer@defaultospec{#1}}%
  ifnum @itemdepth >2relax@toodeepelse
    advance@itemdepth@ne
    beamer@computepref@itemdepth% sets beameritemnestingprefix
    usebeamerfont{itemize/enumerate beameritemnestingprefix body}%
    usebeamercolor[fg]{itemize/enumerate beameritemnestingprefix body}%
    usebeamertemplate{itemize/enumerate beameritemnestingprefix body begin}%
    list
      {usebeamertemplate{itemize beameritemnestingprefix item}}
      {defmakelabel##1{%
          {%
            hssllap{{%
                usebeamerfont*{itemize beameritemnestingprefix item}%
                usebeamercolor[fg]{itemize beameritemnestingprefix item}##1}}%
          }%
        }%
      }
  fi%
  beamer@cramped%
  raggedright%
  beamer@firstlineitemizeunskip%
}
makeatother

...
      {defmakelabel##1{%
          {%
            rlap{{%
                usebeamerfont*{itemize beameritemnestingprefix item}%
...