Well, I've already asked, and unless that ruling's been changed, you can sell the horses once you adopt them... I had that in mind specifically when I asked. So you can adopt a wild horse, train it, sell it, adopt a second wild horse, repeat.
And you don't really need 3 stables. You don't even really need 1. All you need is some hay and a water trough, maybe some training equipment, and your horse will be able to attend to all of its needs. The box stall allows for breeding, constantly supplies a food source, can supply fertilizer, and that's about the whole of it. So...Your milage out of a box stall will vary.
As for how hard it is to supermax... It looks pretty doable, particularly if you have aids like the moodlet manager and motive mobile, and you start as a teen. I've been having a test file where my sim started as a young adult, never owned more than 2 horses at a time, is about to become an adult soon... And only has 2 challenges left, both of them competetion-oriented. She's also maxxed the horsewoman career.
