Tag every value of a single multi-value field
you could create a form with multiple file upload fields, and on submit have it create three nodes, adding the file information to a file field in the new node


Form API


you could take a look at Multi Node Add Module:



  This module provides a new interface for creating nodes where the
  users are able to select the fields what they want to use for creating
  the nodes. It is designed to create multiple nodes in one round, it's
  possible to add more node input rows on-the-fly.


I have never used it, but it appears to do the first suggestion automatically for you. Even if it wasn't perfect, you could modify it into a custom module using it as a framework to build upon


using a temp content type that is only used as a form (for simplicity), or creating a custom form via code or module, you could create a Rule acting upon submit to create three of the actual image nodes, using the field values from the temp content type. If Multi Node Add doesn't work, this has an advantage of the easiest (though probably least performance) way of accomplishing it of these options



  The Rules module allows site administrators to define conditionally
  executed actions based on occurring events (known as reactive or ECA
  rules). It's a replacement with more features for the trigger module
  in core and the successor of the Drupal 5 workflow-ng module.