Mobile 3.1 is compatible with RiOS 8.5 but you won't have some features such as SMB2, Outlook anywhere (RPC/SSL), the neser SMC interface, etc.
Mobile 3.1 feels like Rios 6.0 while Mobile 4.x feels like RiOS 6.5. Rios 7 features (i.e. Citrix CDM) and Rios 8 features (i.e. Citrix Multi-Port ICA, SMB3 (8.5) are not yet supported on Mobile.
In fact Mopbile is roughly 2 years late on Rios in terms of feature support. They haven't released a single version for 6 months, not even for bug fixes.