The horse's top 3 skills are what are needed for 100 blup. So go look on your horse's page and click on the Genetics tab. Then find the 3 skills that have the highest, individual gp. Those are that horses top 3 skills. No one skill is more important than the other two.
You can find the top 3 skills for each breed by going to the directories and then clicking on each of the breed links. But those don't hold true for every horse due to a myriad of things. That's why looking at the horse's genetics tab to find the top 3 skills is the most accurate.