Haven't really been playing that much since I hit plat a couple months ago. I'll just sign on to make sure I don't decay but just wondering how people are seeing his build path now with all the new items. If I start Triforce, which armor/health item should I get right after? Is it still gonna be Randuin's, or is there another item people are seeing as stronger?
i'd still think the core damage items for jax would be 
and 
, then build tanky. I haven't tried it yet, but i imagine that 
would be more useful on Jax than most other champions; if you're about to be bursted, just turn on your dodge, and the remaining AoE will activate the Sterak passive.
Not a Jax player, nor a plat player, but what I've been doing for my armor+health item is: Randuins if the enemy adc is one that builds IE in to PD or Shiv (typical adc) Dead Mans Plate if the enemy adc is one that doesn't (e.g. Vayne, Ezreal, Corki, etc...)
So I get the 10% crit damage reduction against an ADC that's critting a lot, and I get more health and a damaging and mobility passive if I don't need the crit damage reduction. I haven't tried Sterak's Gage or Titanic Hydra yet, but neither is an armor item so I don't know if that's what you're looking for. Sterak's increases BASE ad so it will scale nicely with TF, it could actually be really good on Jax.
Obviously it's not always that simple. If they have a tryndamere or master yi for example, I'm probably going randuins even if the ADC is Ezreal. If they're AP heavy with adc Ezreal, I'm prioritizing my MR before DMP if I build DMP at all.
