Aokiji can generate, manipulate and turn into the very element all three fight with. That alone puts him above Toshiro and Gray. Also add the fact that Aokiji is the most experienced combatant, and it's his win all day.

Simple analysis shows this to be true.

Gray is, at least most often, the weakest of this group by far. His magic creates ice, but has little control over it, and can only make forms that he can easily visualize. One might argue that, with his father's demon slayer magic, he was more than a step above, since he could instantly freeze large objects and other people.

That might be enough to take out Toshiro. He can control ice, in large volumes, but he is limited in his capacity to generate it, not to mention incapable of doing it over long periods of time. Still, against Gray most of the time, he'd probably be the easy favorite, given his incredible speed and strength, combined with the versatility of his usage of ice.

But Aokiji blows them both out of the water. First off, just the fact that he's a Logia user who is literally composed of ice means they can't really do any harm to him. Even assuming they had something akin to Haki, Aokiji is one of the fastest and strongest characters in the One Piece universe, and therefore more than a match for Gray's speed, and probably even for Toshiro's. But the kicker is that he can instantly freeze pretty much anything he wishes AND has absolute control over ice, and can produce incredible amounts of it. He's the easy winner.