You haven’t said whether Y is Hermitian semidefinite. But let me afford you every benefit by assuming that Y is the identity matrix. Then your resulting 3 by 3 matrix, ((Voltage) * (Voltage)’ * Y) has (i,j) element being a product of variables, Voltage(i)*Voltage(j), and those elements are non-convex for the off-diagonal elements (i.e., i \ne j).