Since the area of a triangle is given by 1/2 base*height, the area of the triangular-shaped house of cards that Zion made is 1/2*40*20 = <<1/2*40*20=400>>400 square centimeters.
If two of his friend also built similar shaped houses, the total area of their triangular-shaped house of cards is 400+400=<<400+400=800>>800 square centimeters.
Together with Zion, they all built triangular-shaped houses of cards with a combined area of 800+400=<<800+400=1200>>1200 square centimeters.
#### 1200