I'm not sure that this page is still supposed to be out there, but I found the older version downloads here (2.5 and earlier).
....and this blog has some links of the source on github at the end of the article.
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