The deciding physical traits in a racehorse are the length of hip, as hip length is equal to pure power potential, and the ls joint placement. Length and depth of loin would also play a large role. All three horses are exceptional in these areas.
Not all crooked legs are created equally. Seabiscuit being over at the knee is irrelevant. Had he been calf-kneed...that'd have been a different story.
All three horses are very similiar in other conformation traits...downhill built, high hock and stifle sets, straighter hind leg with more length to the fibia than tibia, etc., etc.,
The biggest difference between the three is the ratio of fast twitch to slow twitch muscling.
Most racehorses have the basics in their build. What can't be determined until you start training them is how they will react to the training, to the racing itself, will they be easy on themselves or hard. Are they extroverts, introverts? Will they hide their anxiety or act out? Will they relax during a race, or get themselves so wound up that they are done before the gate opens? Are they competitive? Are they overly competitive? Are they completely underwhelmed by the whole thing?
A good portion of racehorses are ruined by straight up bad training and management. Not fed right, not rested enough, not turned out enough, not trimmed or shod right, pushed too young, pushed too hard, and on and on and on.
Most times, a great horse is a great horse despite its humans, not because of its humans.