Julia 0.6, 16 bytes
!x=x⋅x∈2x.*x
How it works
Let x = [a, b, c].
x⋅x is the dot product of x and itself, so it yields a2 + b2 + c2.
2x.*x is the element-wise product of 2x and x, so it yields [2a2, 2b2, 2c2].
Finally, ∈ tests if the integer a2 + b2 + c2 belongs to the vector [2a2, 2b2, 2c2], which is true iff
a2 + b2 + c2 = 2a2 or a2 + b2 + c2 = 2b2 or a2 + b2 + c2 = 2c2, which itself is true iff
b2 + c2 = a2 or a2 + c2 = b2 or a2 + b2 = c2.
Julia 0.6, 16 bytes
!x=x⋅x∈2x.*x
How it works
Let x = [a, b, c].
x⋅x is the dot product of x and itself, so it yields a2 + b2 + c2.
2x.*x is the element-wise product of 2x and x, so it yields [2a2, 2b2, 2c2].
Finally, ∈ tests if the integer a2 + b2 + c2 belongs to the vector [2a2, 2b2, 2c2], which is true iff
a2 + b2 + c2 = 2a2 or a2 + b2 + c2 = 2b2 or a2 + b2 + c2 = 2c2, which itself is true iff
b2 + c2 = a2 or a2 + c2 = b2 or a2 + b2 = c2.