If you are looking to download the. ArcGIS Desktop or Server 10.3.1 software, that is usually available to your software administrator (if not you) thru the https://my.esri.com site, if in the U.S. It may be different if in another location (possibly thru a distributor).
Rebecca...I always get ours through our Library that handles the license for the University. At least in Canada, that is usually the way it is handled...never thought about other countries. Thanks
If you are referring to the ESRI JS API 3.11: