Arm I: d4T + ZDV placebo + DLV + IDV Arm II: ZDV + d4T placebo + 3TC + IDV Arm III: ZDV + d4T placebo + DLV + IDV Treatment on all arms is given for 24 weeks. [AS PER AMENDMENT 12/19/97: The study is no longer partially blinded, and placebo agents are no longer given; treatment duration is now 48 weeks.] [AS PER AMENDMENT 8/24/98: study duration is now 96 weeks.] Rollover patients from ACTG 306 with greater than 500 HIV-1 RNA copies/ml previously assigned to ZDV/3TC are nonrandomly assigned to Arm I; those previously assigned to ddI/3TC or d4T/3TC are randomized to Arm II or III. Non-rollover patients are randomized to Arm II or III. Rollover patients from ACTG 306 with 500 HIV-1copies/ml or less continue on their previously assigned regimen [AS PER AMENDMENT 12/19/98: current regimen must be ZDV/3TC, ddI/3TC, or d4T/3TC.] for the study duration or until an increase occurs. If this increase occurs, patients previously assigned to ZDV/3TC are nonrandomly assigned to Arm I for the remaining study weeks, while those previously assigned to either ddI/3TC or d4T/3TC are randomized to Arm II or III for the remaining study weeks. Patients who received ddI/d4T or ddI/3TC in ACTG 306 are stratified by whether patients received monotherapy or combination therapy during the first 24 weeks [AS PER AMENDMENT 12/19/97: 48 weeks]; [ AS PER AMENDMENT 8/24/98: 96 weeks.] of ACTG 306.