In Drupal 7, What is a good approach to copy the image from one imagefield to another imagefield?
You should only use one image field, and then setup 3 image styles at:


  admin/config/media/image-styles


For each image style you can define a number of different actions, like cropping. When displaying the image you can use theme_image_style()

This is a part of Drupal core and can be extended by e.g. ImageCache Actions.