None taken, they're the original Three Musketeers of Planetside.
As for why "TGD", I reckon it's either "TerraGen Document" or it's because TG2 is the 4th incarnation of Terragen (preceded by "Terragen A", which Matt coded on the Amiga, "Terragen B" otherwise known as Terragen 0.x, and "Terragen C", an attempted conversion of the TG 0.x UI to C++ from Visual Basic which never saw the light of day). The rendering engine used in TG2 is known as "TGD".
- Oshyan