The HDP Sandbox a single node cluster of Apache Hadoop® and its extended Apache ecosystem. The whole purpose of sandbox is to provide a quick and easy personal desktop environment to get started learning, exploring, developing, testing and trying new features.
If you really want a 3 node cluster then i will suggest you to not use Sandbox (as it will cause more effort and will consume much of your time), rather you should create a fresh 3 node cluster setup on your own.
I could add slave node using sandbox. But on restart the entire services in the slave node became disabled.