Give advice on how to fix Google Webmaster HREFLANG errors

I have an Online Shop that is run in three languages (german, english & french), using three main domains (.de, .[login to view URL] & .fr), all hosted on the same server. Google Webmaster Tools are reporting errors to me with the HREFLANG implementation on my site - this same error is reported for all three languages (de, en & fr): "International Targeting | Language > 'en' - no return tags URLs for your site and alternative URLs in 'en' that do not have return tags." Example Error: Originating URL: [login to view URL] Alternative URL: [login to view URL] There is a number of these errors, and they are repeated across the three languages. I require specific advice on how to fix this specific problem, and potentially further advice down the track on how to improve the organic ranking of the site.