C++ :: Overloading Binary And Assignment Operators In Vector Class

I am making a vector class and am having some problems creating the overloaded arithmetic operators and assignment operators.

Here is an example of my "+=" code as it stands the errors are similar/the same for the other operators except "=" operator which works fine:

Vector3& Vector3::operator+=(const Vector3 &rhs) {
Vector3 tmp = *this;
Set(tmp.getX() + rhs.getX(), tmp.getY() + rhs.getY(), tmp.getZ() + rhs.getZ());
return *this;
}

I have tried a lot of different approaches ad always get the error:

error: passing 'const Vector3' as 'this' argument of 'double Vector3::getX()' discards qualifiers
error: passing 'const Vector3' as 'this' argument of 'double Vector3::getY()' discards qualifiers
error: passing 'const Vector3' as 'this' argument of 'double Vector3::getZ()' discards qualifiers