Viscose is another name for rayon, artificially made fiber using natural source (wood, cotton fibers). The fabric made of viscose/rayon is absorbent,dyes well and drapes well. It is also referred to as "artificial silk" for a while before the name was discontinued due to the negative association from the word "artificial".
Modal is also an artificially fiber processed from natural fiber, beechwood. It is essentially a type of rayon.
Polyster is totally synthetic, meaning there is no natural fiber to begin with. It is not absorbent.
You can find these from wikipedia.org, which is a great source for a lot of information.