Aizan is right about the diopters for the Nikon F3.
Let me add that all the 6x7 and 6x9 Fuji cameras starting from the G690 BL -- it means that only the original G690 from 1968 is different -- are equipped with a 19mm round viewing eyepiece thread, just like the following Nikon models: FM3A, FM2, FA, FE2, F3, F3AF, FM, FE, F2, F and Nikkormat series. All the Nikon viewfinder accessories designed for the aforementioned cameras are therefore compatible with the big Fujis.
I personally recommend the Nikon DR-3 right angle viewfinder attachment when using the camera low on a tripod.
Cheers!
Abbazz