v3.0.0 is the first API stable release of the v3.x series. We do not expect any future API breaking changes.
For C++, Java Lite and Objective-C, source level compatibility is guaranteed. 
Upgrading from v3.0.0 to newer minor version releases will be source compatible.
For example, if your code compiles against protobuf v3.0.0, it will continue to compile after you upgrade protobuf library to v3.1.0.