MPEG-3 is the designation for a group of audio and video coding standards agreed upon by the Moving Picture Experts Group (MPEG) designed to handle HDTV signals at 1080p in the range of 20 to 40 megabits per second. MPEG-3 was launched as an effort to address the need of an HDTV standard while work on MPEG-2 was underway, but it was soon discovered that MPEG-2, at high data rates, would accommodate HDTV. Thus, in 1992 HDTV was included as a separate profile in the MPEG-2 standard and MPEG-3 was rolled into MPEG-2. (cited from Wikipedia)
Unless your video dates to sometime before 1992, you do not have mpeg3 video. I've never seen it, nor seen any software for working with this format that was never released to the public.
Download MediaInfo here: http://mediainfo.sourceforge.net/en Install it, and open the file with it. MediaInfo will tell you the exact codecs used in your video. I find the Tree view within MediaInfo to be the most informative.
You can copy and paste that info here in your thread and we will be better equipped to help you.