reconsider A = C, B = D as Subset of T ;
A1: A is open by Th1;
A2: B is open by Th1;
A3: A is closed by Th2;
A4: B is closed by Th2;
A \/ B is open by A1, A2;
hence C \/ D is Element of OpenClosedSet T by A3, A4, Th3; :: thesis: verum