I work with documents which need to be greater than 3m and really would like to continue working with OO / LibreOffice but can’t seem to see how documents can be legitimately created to be greater than the 3m
No, it was restricted, because the zoom cannot be smaller than 5% and that is the zoom for full page view of 3m.
The precision of drawings are internally 1/100 mm, 0.01 in UI. I doubt, that you need that precision on a 3m drawing. Have you tried to work in 1:10 scaling?