Horse legs finish up by 4. Backs finish at 5. So if you're racing a horse at 3 there thundering around the track on legs that have not finish solidifying.
Ruffian was 3 when she went down.
People are free to believe what they want. There are vets who morn over horses that are injured in these races, but will patch them up, give them something for the pain and help them heal so they can get out there and race again. Knowing that the horse is still a young, growing animal. There is a reason horse have to be at least 5 to compete in endurance races (4 for the shorter limited race). At 4 the legs should be finished firming up. At 5 the back (last thing to finish growing) should be ready.
You can race at 4, but in the TB Derby racing industry training begains long before they ever race, so if you are going to give the horse the best chance to be completely sound you limit the amount of stress you put on the legs while still forming. That means at 4 whan the legs are ready you can start the hard training. Otherwise, to race them at for you'll have them technically "racing" (watch how these horses are run for training) while they are 3 so they can race at 4.
As for all these wonderful grooms/trainers/vets/handlers/owners, etc.... who love and care for these horses. If I took your horse at the age of 3, provided it loving care and attention. Provided it with a perfect diet. Gave it the best medical attention money could buy.
BUT....I also raced it every day to build up it's muscles and endurance. Pushing it a little harder as it's contioning improved (or perhaps I should say I gave it workouts every day....that sounds better, but pounding around the track as fast as you can has the same effect whether you're racing or training). And when it was old enough (we might have started "training" at 18 months, but we'll be nice and say 24 months), at 3 years old I put your beloved horse in races with other 3 year olds. All still waiting for their legs to finish growing. So for over a year I've subjected your horses legs to as much trauma as I could without making it break down (I have a vet there to check it regularly, after all we want to be able to race). Now I've put your horse on the track and we'll push it to run it's heart out as fast as it's still growing legs will carry it. And as long as your horse holds up and looks like it has potential I'll continue to put it into the races (hopeful of winning some money). Don't lose track of the fact that I'm "love" and "care" about your horse, but I'm putting it's still forming body through a significant amount of stress.
Now, how much do think I really love your horse?
Look at Ruffian. Do think for one minute that all the people connected with that lovely filly didn't love her? This horse that was a proven winner. Setting records (some of which might still be standing) and believed by some to be able to win the big races dominated by the colts. I can assure you there probably wasn't a dry eye among the people connected with Ruffian when she went down and had to be put down. BUT they still raced her, like they do all these horse, at an age when she should not have been raced. And she had not gone down, and had won that race (which I don't think anyone doubts she would have won), they would have kept right on racing her.
I'm not saying none of these are good people. Being a good person doen't mean you truly care about a horse. If it was YOUR 3 year old that they were doing this to, what would you say? How good would they be then, with respect to the well being of your horse? They would never do this to my 3 year old, because they don't have her best interest at heart. It's still about the money for all of them. It's their job, their livelyhood. They depend on it.