Why does this function not compile?

its because of the line:

Function CompNormal:Float[] (vertex1:Float[3], vertex2:Float[3], vertex3:Float[3])

You can't have Size initializers in the method or function description. So 'blah:float[234]' is illegal.

try:

Function CompNormal:Float[] (vertex1:Float[], vertex2:Float[], vertex3:Float[])

And then check if the array's are of proper length inside the function.

If( vertex1.Length < 3 Or vertex2.Length < 3 Or vertex3.Length < 3 ) Then
	Throw "Arguments are not of the expected size."
End If