Based on some simple research, libamfx does not support AMF3 and amf3cplusplus is not a complete implementation of the specifications, although it may work for your needs. There do not seem to be any libraries with pure C, so the next place to look is the C language extension.
There is a full functional AMF implementation in C for PHP ( AMFEXT ), a C implementation for Python ( PyAMF ), one for Objective-C ( Cocoa-AMF ) and one for Ruby ( RocketAMF ). However, none of them will be a rollback solution.
Your best bet is to try amf3cplusplus and see if it does what you want. If this is not the case, RocketAMF will probably be the easiest to use as a starting point in terms of readability and replacing language-specific code with C ++ data structures.