The three-character suffix is actually a hex value in the range 000 to FFF. This is a temporary file that FM writes during various processes (such as Autosave). It first creates the file with the odd suffix (i.e. xxxx.fm.0FF), and when it completes the writing task, it deletes the specific .fm version and then renames the odd suffix (i.e. xxxxx.fm.0FF) version to the .fm version it is replacing.
If there are any issues (network, file space, phase of the moon, etc.) during this three-step shuffle, FM may not be able to complete the save of the final version. FM then typically issues a warning message: "The document was saved to a temporary file, but FrameMaker cannot rename it to have the correct name. The newer version has an odd suffix."
If it's an old file, then you can delete it. However, if it's very recent, then it *may* contain edits that didn't make it to your file.