Third item, sure. I can see that. Armor pen is much more useful, the marginal benefit is much larger, mid game to late game. Rushing the item? No. Second item last whisper build is a horrible idea, imo. You're not getting the gold benefit out of it now that you would building it mid game and spending the 2700 that it cost early game on a Qss, lifesteal, cdr, flat armor pen, etc.
edit: Also, as an ADC, you're not to be focusing tanks :^) You're to be focusing the squishies, therefore no need for major armor pen unless it becomes quite the issue. Two tanky champs such as Shen/Gragas doesn't mean much if you're kiting and have a magic damage mid lane; the more armor they build against you, the less magic resist they have for your mid lane, and vice versa. It's a very situational item that shouldn't be rushed in each game with tanks.. Especially considering there are things such as health tanks, sustain tanks, etc.