Number one is the only correct choice for what you are trying to say. The waxy oil keeps, or prevents, the ears from getting too dry. Dry is an adjective, and "too" modifies "dry." You can't use "too drying," because "drying" is based on an action verb (it's a gerund), but you could say "drying too much." The last sentence is grammatically fine but has a different meaning than what you want to say.