Chromosomes usually appear in pairs. If there are three chromosomes where there should be two, what kind of mutation is this?
This is a genetic mutation called Nondisjunction.
When chromosomes don't properly separate during meiosis, there will be an uneven number of chromosomes in pairs. It could be trisomy or monosomy. Trisomy is when you have 3 chromosomes in a pair and monosomy is when you have only 1 chromosome in a pair. These cause genetic diseases such as monosomy-21 (21th pair in the karyotype) which is also known as down syndrome.