It makes more sense to me that instead of the three spren that the three Bondsmiths are bonded with mimicking Greek mythology that they are three powerful and unique spren, each being a Sliver of a Shard that has an influence on Roshar, meaning that there would be:
A spren from Honor: This would be the Stormfather. This, of course, is already confirmed in the book.
A spren from Cultivation: My best guess for a powerful unique spren that's a Sliver of Cultivation would be the Nightwatcher. This theory is supported by how just as Syl refers to the Stormfather as her father, and the Stormfather refers to Syl as his daughter, Wyndle (The spren Lift is bonded to) refers to the Nightwatcher as his mother.
A spren from Odium: Since the spren didn't start to bond with men until after the desolation a began and the ten heralds formed the oathpact (which is when I presume that Odium entered the area of Greater Roshar, and began to influence Roshar), it is possible that there is a Sliver of Odium in the form of a powerful spren that can bond with people. But like other Odium Sliver spren (storm spren) I would presume that this powerful Sliver of Odium would be just as hostile to people as Odium is. This theory seems weak, however, as the Knights Radiant were an order dedicated to fighting the Voidbringers, and it is though that the Odium is responsible or somehow connected to the Voidbringers.
Of the three theories, the last is the least likely, due to the contradicting ideas pertaining to Odium and the Knights Radiant. Despite this, I have no better ideas. Anybody else have any suggestions?