Fighter is now confirmed to get 3, so there is probably only one other or maybe two, if the number is over 25 instead of just 25. There is a number of classes that could use some subclass love, namely bard, druid and ranger. An argument can also be made for barbarian and sorcerer.
When it comes to bard, we know that glamour is in and blade bard appeared among the revised subclasses and has high probablity to be in the book. But i don't think the college of whispers was popular to make it (and neither wasn't the college of satire).
Druid is similar, we know that circle of dreams is in and there have been hits of circle of shepherd being in, but college of twilight wasn't popular enough to make it.
When it comes to ranger, we have the horizon walker confirmed, but have no idea what the second might be. The deep stalker seems a high possibility, but primeval guardian seems doubtful.
Barbarian is something i find hard to predict. Barbarian got a subclass in SCAG, but the battlerager wasn't very popular, so it can be argued that the barbarian could use more subclass support. Ancestral Guardian was among the revision, so probably its in. There were people who really liked storm herald, but less who liked the zealot?
With sorcerer we have both two subclasses confirmed shadow and divine soul. Unlike the barbarian, its SCAG subclass was well received and has a possibility of appearing among the four SGAC classes in XGtE. So the one hand it could be said that sorcerer probably might not need to have another subclass on this book. But on the other hand there are mixed attitudes and issues with the sorcerer, so offering more to the sorcerer can be a decent step. Here in enworld the stone sorcerer seems to be a wanted addition, but this and the phoenix and sea were far less interesting to people than the Favored (or now divine) soul. Phoenix was probably the one with less interest, while Sea seemed to have some following, but concerns about it being too close to Storm, might push it back.
Of course there are other classes that might get the third. Cleric is a possibility, so is Warlock. Those classes have gotten a lot, but have still some thematic room that people are expecting to fill. Warlock is perhaps the most interesting case here. The Celestial patron is confirmed. Hexblade is suspected, but something to consider is that among the Warlock UA were a number of Seeker patron invocations. Was it a hint that its going to be in the XGtE? Were they in just because the subclass was shown earlier in UA and they wanted to add some non-core book Patron ones, but didn't want to add invocations specific to a subclass that had appeared in another class? Or are we actually getting three new Warlock patrons. The Raven Queen one is probably out, because it got pretty negative or rather meh reactions to its features. Might Undying patron make its appearance among the SCAG 4, but i'm not sure, while there were people who seem to like or expect it, the general attitude towards the patron seemed to be "not so interested." Also the fact that there were no undying patron invocation in the Warlock UA, can also mean that its not going to be among them.
Stopping for a moment on Cleric. Forge confirmed, Grave extremely possible. And lets be honest, although the features of the Protection domain weren't the greatest, it might have gotten through, because a lot of people were interested in Protection domain.
So there are possibilities, some more probable than others. I think its highly unlikely that monk, paladin or rogue are getting a third one. Or fighter a fourth...
