Although in humans there are 22 pairs of autosomal chromosomes, only three different chromosomal trisomies are commonly seen in newborns. of the remaining 19 autosomes, many trisomies have not been seen in newborns. why not?

trisomy for the other autosomal chromosomes is often lethal, and the affected embryos are miscarried. trisomy for these other autosomal chromosomes occurs so rarely that it has never been documented. trisomy for these autosomal chromosomes has no effect and therefore would never be noticed. these autosomal chromosomes do not contain the same type of dna or protein that makes up chromosomes susceptible to trisomy.

The answer is; Trisomy for the other autosomal chromosomes is often lethal, and the affected embryos are miscarried

Most trisomy that occurs involves sex chromosomes. This is because the Y chromosome (that codes for male gender) carries very few genes to have enormous implications in males in case of mutations or nondisjunction. In females, X chromosome occurs in pairs where one is silenced. Trisomy in these is not lethal. Trisomy in other critical chromosomes result in improper development of fetus and is unable to develop normally to birth.  

this is true, assuming that the allele is completely recessive and incomplete dominance is not a factor.