The cube roots of unity are
1,w,w2
Where w3=1
Now
If the form an equilateral triangle they must satisfy the following relation
z21+z22+z23−z1.z2−z2.z3−z3.z1=0
Thus,
1+w2+w4−w−w2−w3
=1+w2+w3.w−w−w2−1
=(1−1)+w2−w2+(w−w)
=0
Hence, they form an equilateral triangle.