I think you will find that the square file profiles will not fit the holder on the FG3.
There have been people remake the file holders on a Stihl FG2, which is similar, but bolts to a workbench and includes a chain clamp, as opposed to the FG3 which clamps onto the bar.
The FG2 gives me great round filed chains, although they are NLA in the US according to other reports here. I would recommend one of those.