Third-order interelectronic-interaction correction to the energies of (1s)²2s and (1s)²2p(1/2) states of high-Z lithiumlike ions is evaluated within the Breit approximation in the range 20 ≤ Z ≤ 100. The calculation is carried out using both the relativistic configuration-interaction method and perturbation theory. The correction is shown to be important for the comparison of theory and experiment. (C) 2000 Elsevier Science B.V.