It's doomed as in they'll die with most bad leg injuries. Horses cant support themselves on 3 legs. They're just too big. There was a foal with 3 legs that lived for a couple years and made its round on social media, but it was eventually put down because it just grew too large. Poor thing should have been put down immediately because the life it lived was of terrible quality despite getting specialized care.

I've seen a few horses get put down for a bad leg injury. Almost had to put down my childhood mare after she fucked up her leg in a fence. She made a full recovery, but it took almost a year for it to heal. Her leg got caught in the wire fence and it got shredded to the bone. It was awful, but she was able to stand on it enough to keep herself upright and all that, so we decided to try to get it healed and were miraculously successful