The full form of MPEG-3 is Moving Picture Experts Group Phase 3

Moving Picture Experts Group Phase 3(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

It is important to note that this specific standard no longer exists, and that the standards set forth within it are now a part of MPEG-2. When the group originally took on the responsibility for setting the regulations for HDTV in 1992, they were simultaneously working on the other standard. It was discovered, however, that MPEG-2 encoding could accommodate HDTV signals as well, so the new standard was therefore discontinued.