CORE3D is allegedly a file format used in 3D model virtual files, but it is hard to pinpoint exactly what program uses it or any other information about the format. A description
CORE3D - Coreona 3D Coreona 3D Virtual File(.core3d) (complete with Coreona 3D repeated -- the program so nice they had to name it twice?) is present verbatim in numerous sites listing file formats, seeming to be copied and pasted from some place, perhaps the Wikipedia list of file formats which has this from an edit from a raw IP address in 2010, which linked it to something called Old Oak Productions, whose Wikipedia page was later deleted as spam.
This "copypasta" text dominates searches, making it hard to find any primary source that might have more information. Thus, there is alleged (but not proven) to exist a program called Coreona 3D (or maybe Corona 3D if it's misspelled in the widely-copied text) that has virtual files with a .core3d extension, and is possibly produced by a company called Old Oak Productions. Any information that sheds more light on this would be useful.