Which term describes the chromosomal abnormality of having three copies of a single chromosome

Trisomy

Explanation:

Trisomy is the presence of three copies of one chromosome instead of the normal two. Trisomy usually causes some anomaly such as Patau Syndrome, Edwards Syndrome or Down Syndrome. Many trisomies cause early deaths, but with the advancement of technology and medical treatment, this problem of early death has become ever smaller.

the answer to your question is tooth decay.