In order to blup your horse, train in the 2nd and 3rd skills. Don't train in the highest skill until you get 20 wins.

To add, the other skills aren't a factor in blup, they are just extra skills. Skills aren't factored in when breeding, only the blup and gp at the time of covering.

Training multiple skills at a time isn't technically more or less efficient than training one at a time, unless you're relying on your horse improving in a certain skill to perform better in competitions.

However, different skills have different energy usage. Speed and stamina use the most, I think, Jumping, Gallop, and Trot slightly less, and Dressage the least. Because of this, you might want to mix and match a bit to get maximum energy usage from your horse in a day.
For example, Dressage uses so little energy that you'll often run out of time in your horse's day before you run out of energy if you're exclusively training Dressage. I always train Dressage at the same time as a different skill, so I make maximum use of both time and energy. Usually 3hrs of Dressage, then the rest of the day in a different skill works well for me.

Also, if your horse is finishing the day with 25% energy, that extra 5% isn't enough to fit in any more stamina or speed training, but you can do a half hour of Dressage training with that energy.

Outside of these, there's not really any advantage to training at one skill vs training in multiple skills, but most people seem to train one skill at a time for convenience.

To add, I'd also keep in mind the effect training has on competition difficulty (if you're trying to BLUP your horses). The competition's main skill is also the difficulty determining one, meaning if you train up that skill in your horse (e.g. stamina for XC comps), you're likely going to find winning more difficult because your horse will be entered into harder comps.

If you're blupping, I'd generally recommend training your horse's secondary and tertiary skills, then entering competitions, and then training the primary skill, to make winning easier.

If you're not blupping, then you can ignore all of that and train however you please.

(Oops, I just now saw this is what tigrelily posted, but I'm going to post anyway just to add on to what they've said)

I have a question, can you get an aging point when a foal wakes up?

Yes, foals also have a chance to find aging points when they wake up. The only horses that can't find APs are those with Morpheus' Arms

Please be sure to start your own topic if you have a question

Thanks all! Locking up