.MJ3 File

The mj3 file extension were frequently used on files found on CD-ROMs with games based on id Software engine, such as DOOM or Heretic.

The purpose of  .mj3 files was to allow the user of one game, to buy online the other game. So for example on Heretic CD-ROM, there was mj3 file for DOOM game.

How to open:

These old files were used to purchase the game online, probably acted as some sort of link or ticket.

How to convert:

A conversion for .mj3 file seems unlikely and seems to be not possible.