Warlords is primarily for Auto attack based champions who have a general higher per auto damage than others. Examples: Cait, Draven, Tryndamere, Yasuo. Basically anyone that builds crit or has an auto attack modifier that benefits from it. The sole exception to this is vayne as she doesn't need fervor and only needs items to set up her W. Though that doesn't mean this is an iron clad rule as you're free to take fervor or warlords as you please and it'll typically have successful results either way.
Fervor is built on champions outside of the first category who do alot of on hit damage as that generally involves a high attack speed which works well with fervor. Examples: Kayle, Kogmaw, Aatrox, Jax, Yi.
Undying is most taken on characters who build tank but focus more towards being a damage tank over a raw tank. It gives damage and sustain based on % Max HP so it's good for a brawler type tank. Examples: Shen, Nasus, Garen, Darius, Sion.