Patients with prior AZT experience only are randomized to receive either d4T alone or AZT/3TC. Patients with prior ddI experience only are randomized to receive ddI/AZT or ddI/AZT/3TC. PER AMENDMENT 8/27/96: The study has been extended 6 months and treatment will be available until March 15, 1997 at the latest. Each patient will have regularly scheduled 12 week safety visits during the extension period.