My video is 3 minutes long and when I export it says 20 mins

When this comes up it is usally a stray frame or section of a clip that gets put past the end of the video.  

Solution 1:  Press the "\" key.  Your 3 minute video should fill the timeline.  If it is crowded to the left, there is a frame or clip toward the right.  It may be on a track above track 1.   

Solution 2:  Use the work area bars, mark the 3 minute section and check the "Work Area Bars Only" box at output.