Final answer:
An organism with three sets of chromosomes is a triploid, and one with four sets is a tetraploid. This condition is known as polyploidy, which is rare in animals but common in plants.
Step-by-step explanation:
An organism that has three sets of chromosomes is called a triploid, while one with four sets is called a tetraploid. Most organisms, including humans and many plants, are normally diploid, meaning their somatic cells have two sets of homologous chromosomes, with one set inherited from each parent. Polyploidy refers to conditions where an organism has more than two sets of chromosomes. Polyploidy is quite rare in animals but can be quite common in plants, and polyploid plants often exhibit increased size and robustness when compared to their diploid relatives.