The way I see it, the big dogs fighting for that 3rd spot are (as of recent) Krom, Roy, and Lyn.
Krom being the latest protagonist, and also from a very successful title seems to have a bit of an edge. Working against him is his similarities to Ike, but he still has a bit of a unique fighting style to offer.
Roy was in Melee and was planned for Brawl, and is among one of the most popular candidates for Smash 4. Is part of the latest game as DLC that requires Marth (his fellow Lord from Melee). Working against him is the fact he was a clone, but that can easily be changed, or be at Ganondorf and Falco levels.
Lyn was an Assist Trophy in Brawl and the only non-Sticker and non-song reference to a game other than Marth's or Ike's (with Ike's being the biggest contributors). Is part of the latest game as DLC and is given a new dual-wielding gimmick. Also represents the first Fire Emblem to leave Japan's shores. Working against her is overall popularity and her stiff competition, no innuendo intended.
Cuts are unlikely this time around unless time becomes an issue again, so it's safe to assume that both Marth and Ike will be sticking around. Especially with Marth's likeness playing a key role in the latest game, as well as being the first DLC character and Ike appearing in near every DLC Chapter in the latest game, and is possibly going to be DLC himself soon.
But who of the three is likely to make it? Would there even be a 4th character?