Millman's Theorem 3-Phase, 3-Wire, Star (Y) Unbalanced Load
The way you are doing it is correct, can't find any mistakes. Just be careful and use the phasors for voltages in the last equation. Also, note that \$V_{ao}\$ is actually the line voltage i.e. voltage across the line A and unconnected neutral line. Similarly, for \$V_{bo}\$ and \$V_{co}\$.