See the Gems page for prices in other currencies. There are generally two ways to acquire the third Builder's Hut: It will use half of your initial allotment of 500 Gems.
However, you do not need a Builder to place Decorations or Traps. They are also required to place and upgrade Heroes. You obtain two Builder's Huts from completing the tutorial, with another three available for purchase by using Gems.
'Nothing gets done around here without Builders! You can hire more Builders to start multiple construction projects, or speed up their work by using green gems.'
