Lucario cannot get the Pure Power ability.
Lucario can only get the abilities Inner Focus, Steadfast, Justified (hidden), and Adaptability (mega only).
Unless you're talking about Alpha Sapphire, Lucario isn't even in Sapphire (or any 3rd gen game).
You can't have three Pokemon in the daycare, either.
As for the odds of passing abilities, the only way to increase the odds is having the mother have the ability you want. If you made Medicham the mother, you'd only get Meditite eggs anyway.
So in conclusion, you cannot and will never get a Lucario with Pure Power unless you hack.