Hi Ela,
The app should work in online with relay server.
If you have an existing relay server you can configure it with SMP 3, else I would suggest you to start with reverse proxy.
Regarding the question on configuring relay server with SMP 3, you can follow this guide: How-to-Guide for Reverse Proxy and Load Balancing in SAP Mobile Platform 3.x
Regards, Midhun
SAP Technology RIG