I am unable to find HDP 2.3 download
On the Sandbox Download Page, there's a "Hortonworks Sandbox Archive" expandable list under the "Sandbox in the Cloud" section. You can find the 2.3 version of the Sandbox there.
See this https://community.hortonworks.com/questions/22647/download-link-sandbox-hdp-232.html
If that does not work then try this https://community.hortonworks.com/questions/6273/hdp-sandbox-archive.html