NIH listing of drugs

The reverse transcriptase enzyme is very nonselective and recognizes AZT as an Thymidine This drug gets incorporated into the growing DNA chain and as was the case with acylovir elongation comes to a halt and the virus cannot replicate.
3-TC or epivir works in a similar fasion replacing cytidine in the growing DNA chain. There is a synergistic effect with AZT, as the virus is generally unable to mutate in ways that alow it to resist both drugs.
Ritonavir (left) is one of the protease inhibitors. By binding to and blocking the HIV protease this drug prevents the virus from processing the proteins it makes. By keeping the amount of virus low it prevents HIV from developing resistance to the entire cocktail.