Xin Zhao Build
Not Updated For Current Season
I use this build and it is very affective in 3v3 because you can 1v3 there team and win the game for yourself. Also this build will work in 5v5, but u might want to build a beefy item in there for team battles. Beware you most likely will be focused in team battles late game (which is when you are most powerful).
Building according to the game
I usually just add items that I feel like I need and sometimes I rearrange the items in my build, but I always start out with my build all the way up to the infinity edge after that I just buy what ever. For example if there is a lot of beefy enemy champs that keep killing you then I would recommend building the last wisper right after the infinity edge and if there is a lot of squishy enemy champs then build the blood thirster right after the infinity edge.
I use crit runes because it helps a lot early game and i become fed very easily also it helps on getting last hits on minions.