playing a specific interval of a video in mplayer using command line option

man mplayer

reveals:

-ss <time>: Seek to given time position
-endpos <[[hh:]mm:]ss[.ms]|size[b|kb|mb]>: Stop at given time or byte position

so you should be able to do what you want by simple using:

mplayer -ss 56 -endpos 3