How to get specific entries from a custom taxonomy? [on hold]

Using a wordpress based website, I have a custom taxonomy "authors" in which I enter the names of publications authors. In this field I usually enter multiple authors' names such as "Mark Twain, Isabelle Lagard, Sandra Cruize".

What I need exactly is:

1. PHP code by which I can display only the first author in one place at the front end. This means that i need to show the first author (e.g. Mark Twain) only without the tow others.
2. PHP code by which I can display only the second author in another place at the front end.This means that i need to show the second author (e.g. Isabelle Lagard) only without the tow others.
3. PHP code by which I can display only the first author in another place at the front end.This means that i need to show the third author (e.g. Sandra Cruize) only without the tow others. etc...

So, what do I have to do please?