A few possibilities, but you could use absolute positioning - in your html put div 3 within div 1. On div 1, set position:relative, on div 3, set position:absolute. The add top and left settings to the css for div 3 to achieve your desired positioning.
That should do it, but dependent upon your html and css elsewhere you may need to set the z-index as well to ensure div 3 appears on top of div 2.
If this doesn't work, then post a link to your page, or failing that, your entire html and css.