Matt,

I use the Lee Filter 100mm System, the old school style since they just redesigned it.

So the bad news is if you want to use 3c with that system your best bet is to buy the special 100mm push on filter, then the 90mm converter ring for it. As Corran mentioned the ring slips over the outside of the 3c filter, then the push on filter tightens a screw that compresses the ring onto the filter. Lee can not make a wide angle 86mm ring, and the standard ring vignettes big time.

B&H Does sell both pieces, the really bad news is that a new one is expensive, like $250 for both expensive. You may want to check the big auction site for someone unloading theirs. I bought the filter holder that way. Here are the links from B&H:

100mm push on holder:
https://www.bhphotovideo.com/c/produ...t_100_4x4.html

90mm converter ring:
https://www.bhphotovideo.com/c/product/292622-REG/LEE_Filters_FKD10090_100_90mm_Converter_Ring_for.html/c/product/292622-REG/100-90mm_Converter_Ring_for_FK100_Filter

Side note 1: With the 58mm on a recessed lens board, then the Center Filter, then the 100mm push on system, it is really difficult to access the controls on the lens. I use a small screwdriver to do everything on the shutter.

Side note 2: I have not researched if the new 100mm system has a new way of doing this. With ultra wide angles this has been a big problem for Lee, which is why competitors have opted for a 150mm systems, which lee does as well.

Good luck, hope this helps.