You mean Act 3 of the game, or Chapter 3 of the season journey (ctrl j)?
Presuming the latter, you get a gift in your mailbox, which will be an icon on the bottom left of the screen. Click that to claim the gift to your inventory, where you then open it.
Note also, that the Crusader Thorns build is the only build in the game that does not care a whit about crit! The bulk of your damage will come from Thorns, which does not crit. What you want on your items is typically thorns (which rolls as a secondary stat), sockets where appropriate for weapon, rings, amulet and helm, attack speed and cooldown reduction.
Attack speed builds more 2-piece bonus damage buff stacks (and faster). Pair with a fast weapon to build 2-piece stacks before hitting Iron Skin/Reflective Skin for damage bursts. CDR helps with faster rotations.
Also an early priority is farming Greater Rifts for Boyarsky's Chip, which will be a huge addition to your thorns damage. Getting this gem should be one of the first things you aim for.