entecavir, aciclovir, and cladribine comparison

The first two are antivirals and the third one is a leukemia medication. I have my usual questions: 

Can they all be used to treat the same things?

Can new medicines be made by making one more like the other?

There are many medicines and useful chemicals based on purine.

suggested reading

Derpetology: cordycepin triphosphate vs adenosine triphosphate & potential cancer link