Coming from DotA, when to buy defensive items?

Solution 1:

Defensive items as a carry in League are very situational. Since there are a lot of skill shots, the general trend is to think that you should simply position correctly and avoid damage. There are some exceptions, and it's against those exceptions that you should pick up a defensive item in your third slot:

If you're playing against Syndra, Katarina, Le Blanc or any other heavy AP damage champion that will probably deal damage to you if you get close, build an hexdrinker third. As Kindred, since you have your ult to help you survive burst and are heavily attack speed based, it might be a good idea to pick up a Wit's End as an alternative counter to magic damage (it gives a lot of magic resist and the passive heals significantly when you're low HP, which you'll be when forced to use your ult.)

If you're playing against point and click CC you'll need to out play, or a lot of skill shot CC's, buy a QSS third. This let's you clean out of the CC and is often unexpected.

Unlike the most widespread narrative, if you're behind on a carry you should never build defensively, as you won't deal any damage and the defensive options aren't enough to counter the stats you lose from the level difference. Offensive stats are often not as reliant on levels and thus should be prioritized when you've fallen far behind. You'll just need to position better and wait out the enemy skills that can kill you.

If you're FAR AHEAD. As in, you're one-two levels ahead of the solo laners with an extra item, you should always make that item defensive. As you're already on pair with the curve in offensive items, an extra defensive item will make you unkillable, protecting your bounty and making it easier to snowball. Often when you're far ahead, killing you is the only way the enemy team can win, and thus countering their only chance to survive is the optimal move. Finishing a second item Guardian Angel while the enemy jungler has just finished his warrior enchantment makes the enemy team REALLY sad and lets you go for crazier plays to compeltely snowball the game.