ArcGIS Flex API has already retired, and that's why you can't the download link from Esri page. If you still want to find one online, you can try to google "arcgis_api_for_flex_3_7" to see if there is any result.
You can find the Flex Viewer 3.7 available through this github page: Releases · Esri/arcgis-viewer-flex · GitHub
We recommend to move forward to JavaScript or any other Quartz Runtime SDK for developing new application.