Originally Posted by Sir Percivale
Well, for Jax, I wouldn't say Black Cleaver would benefit him more than Trinity Force because of his mixed damage. Trinity Force is perfect on Jax, it really compliments his kit. While Black Cleaver will only procs on physical damage, which not of his abilities do. I am not familiar with Third Strike exactly, isn't it magic damage? I don't see Last Whisper working on Jax either.
You can get it on him but it's really not worth it. It'll help his late game but the problem is he will fall off no matter what and Trinity Force is still expensive. The Sheen part kinda hurts you too, seeing as how you don't get the most out of it. Now, Black Cleaver and Last Whisper do work on Lee Sin though, even then, I'd recommend Last Whisper at best on Lee.
Triforce is viable on Lee, but don't rush it. It's mostly for the Sheen proc, but I find he gets more out of Mallet/Thirster.
Sheen definitely doesn't hurt you on Jax, he uses every stat efficiently and his cooldowns are low enough to basically spam proc it.
Leap Strike is Physical Damage, and so is Counterstrike.
Jax doesn't need much / any ArPen because of his insane mixed damage, though (Weapon Expertise is enough)