Actually this is three level mapping.
Add mapping for class C as you have for class B. Extend mapping for class B to cover Cprop.
Assuming that DatabaseX have Xprop field corresponding to Xprop field in target classes(A, B, C) it should look like this:
Mapper.CreateMap<DatabaseA, A>()
.ForMember(dest => dest.Bprop , opt => opt.MapFrom(src => src.Bprop));
Mapper.CreateMap<DatabaseB,B>()
.ForMember(dest => dest.Cprop , opt => opt.MapFrom(src => src.Cprop));
Mapper.CreateMap<DatabaseC, C>();
Here is working example: https://github.com/st4hoo/Automapper3LevelMappingExample