A(3|4|1)
B(3|8|1)
C(0|8|1)
E(3|4|6)
AB = B - A = [0, 4, 0]
D = C - AB = [0, 4, 1] → D(0|4|1)
AE = E - A = [0, 0, 5]
F = B + AE = [3, 8, 6] → F(3|8|6)
G = C + AE = [0, 8, 6] → G(0|8|6)
H = D + AE = [0, 4, 6] → H(0|4|6)
DF = F - D = [3, 4, 5]
|DF| = |[3, 4, 5]| = √(3^2 + 4^2 + 5^2) = 5·√2 = 7.071