It's called 3-slot spacing because if you imagine the slots are numbered, your graphics cards are in slots 1 and 4, thus there being 3 slots between them (don't get thrown off by the fact the top graphics card commonly takes up two slots):
1 (16x) <- GFX
2 (1x)
3 (1x)
4 (16x) <- GFX
5 (1x)
Edit: just checked the image, the M6F omits a PCI-E 1x slot, although the space is still left for it.