The following page explains that ArcGIS 10.3 Desktop, Server and Engine will contain the necessary PostgreSQL Client libraries. You can connect directly from ArcCatalog to PostgreSQL database from ArcGIS 10.3.
Database clients—Help | ArcGIS for Desktop
The PostgreSQL Client libraries are in the version 9.3.5, and they can be used to connect to an older database, like PostgreSQL 9.2.9/9.1.14 database too. So there is no need for manually installing the PostgreSQL Client in 10.3.
Managing Data Geodatabase
arcgis 10.3