Chewbacca is a Wookiee, and therefore testing the device on him would be pointless; his physiology is completely different from a human's.
Leia is female and much smaller in stature than Solo or Skywalker, so the test would not be as effective on her. She is more likely to die than Skywalker, so her death in the test would prove nothing, and be wasteful besides.
There is no evidence that Vader is the kind of person who bothers to take prisoners. He has a distinct, 'choke first, ask questions during the choking, then dispose of the corpse later' policy when it comes to interrogations. High-profile prisoners, such as Leia, seem to be the sole exception to this rule, and they would be too important to risk. From a film-making standpoint, there's also no reason for the audience to care about a random prisoner, even a Rebel.
Solo is roughly Skywalker's size and build, the same species, the same gender, already a prisoner, and already on his way to Jabba's the second Vader doesn't need him anymore. All that is at risk is a few hundred thousand credits extra thrown Fett's way if Solo dies. That's a very simple write-off.
EDIT: There's another issue I didn't think of. Luke can feel Han and Leia being tortured from light-years away. Vader suspects his son has this ability, but he doesn't know it for certain. There's no reason for Vader to assume that Luke has already detected his torture and is on his way. In fact, his initial decision to confine Leia and Chewbacca to Cloud City--presumably with a Star Destroyer or two hiding on the far side of the planet--would seem to imply that he was settling in for a long-term wait.
It is only after Solo is frozen that Vader changes his mind and orders Leia and Chewie taken to his ship; incidentally, they are taken on a route that leads Luke to stumble across the carbon freeze chamber. As they were already in the chamber when Vader made that decision, this is obviously an unnecessarily circuitous route, designed to lead Luke to Vader. This happens to coincide with Luke's arrival at Bespin.
Vader, in other words, was originally freezing Han not only with the intention of testing the carbon freeze for "the Emperor's prize," but also wanted to inflict a little extra torture on Solo for Luke to sense. Luke's arrival rendered the rest of the plan, to leave Leia and Chewie as bait while allowing the Empire, and probably Fett, to spread the word of their presence on Bespin far and wide, moot, so Vader abandoned it. He may also have intended a watch on Luke's arrival at Tatooine, in case he went after Han at Jabba's palace before coming to rescue Leia and Chewie. None of that mattered when Luke arrived at the planet during the test on Solo.
