I hope if there is an FC3 it's a Yugoloth only. I see a lot of people saying to not focus it solely on yugoloths.
Why? There has to be a valid reason other than "I want other fiends to get covered too."
This is the chance for yugoloths to get the coverage they deserve. We'll never know how popular they are or how demanding they can be if WotC doesn't release a book that covers everything old/updated/new about them!
An FC3 will cover updated yugoloths and will cover and update all yugoloth material from the past editions. It will also have enough room for WotC to make them as interesting as possible. It will allow the authors to use whatever creative sparks they have and pour it all into the yugoloths. Devoting FC3 to yugoloths and "others" is just doing them wrong. It's not even doing them any justice at all!
This is like saying FCI should've been about demons and "others" and demon creations. Or FCII should've been about devils and "others". Why not them? Yugoloths have been around in 1E simultaneously with devils and demons, they deserve the same treatment, not less.
This is assuming WotC plans on even writing one but the FCs have been about one race of fiend and I doubt (well, knowing how WotC likes to change formats *coughPRESTIGECLASSEScough* *coughCITYSCAPEcough* *coughMMIVcough*, I -hope-) they will do yugoloths and "others" in it.