Actually, Digimon World 3 was released in the US before released in Japan, if this wiki article is accurate. Either way, it was released when Digimon Frontier was first airing, so it wouldn't surprise me if it was referencing Digimon Frontier. It can be just an easter egg-like reference. Not necessarily connected in any way. Digimon Frontier, in terms of its connections, plot-wise, with the rest of the Digimon franchise, is quite isolated.
I don't know about the games, but in Digimon Tamers, the Digimon and human can "combine" as one powerful Digimon.
Also, a Tamer is a specific term for Chosen Child. They basically control over one or more Digimon.