A 003 file is the third part of a split archive created by a file splitter and combiner program, such as 7-Zip or Incredible Bee Archiver. It contains the data sequentially following the data stored in a .001 and .002 file. 003 files may also have corresponding files with the extensions .004, .005, etc. depending on the file size.
Split archive files allow users to break up a file that is large in size to several smaller files. This is useful when emailing large files that exceed the size limit for message attachments. Examples of file splitting and combining utilities that create and open split archives include Smith Micro StuffIt Deluxe, 7-Zip, and Incredible Bee Archiver.
Since various programs may create .001, .002, 003, etc. files, the files may be saved in different formats. This means that even though a program creates an 003 file, it does not mean that another program that also creates 003 files will be able to open it. Therefore, be sure to check the software that originally created the file to know which program will open the file.