It all depends about what items your ennemies bought. If they bought full crystal damage or weapon damage, and if you want to stay longer in the fight, yes you have to buy Armor or Shield at tier 3. However, if you want to deal a lot of damages, and you don't care about your HP, you should leave the defense (Armor...) and buy some crystal or weapon items. But it's not really a good Idea. I don't really play Carry because I don't really like it, so maybe I said bullshits, but that's my opinion...