Referencing this link
Portal for ArcGIS versions 10.4, 10.4.1, and 10.5 require additional configurations to connect a survey123.arcgis.com website with a Portal. These configurations are not required for Portal for ArcGIS 10.5.1 and newer versions, as these versions are pre-configured to connect to the survey123.arcgis.com website.
My question is whether or not Portal 10.3.1 is supported using this method - it is not specificly stated or specificlly excluded. - Just doing some troubleshooting on an iOS device