Triple X syndrome is a genetic condition which is only associated with females. Most human cells contains 46 chromosomes. Out of these 46, two determine the gender of the individual. Females have two X chromosomes - and are X X - and males have one X and one Y chromosome, and so are X Y.
Now cell division does not always follow a perfect route and there are chances that mistakes can happen during the production of reproductive cells i.e. eggs and sperms. If the error is an allocation of an extra copy of the X chromosome then females are born with three X chromosomes in their cells. This can be noted as 47 XXX. Therefore, this chromosomal abnormality is not an inherited defect, it occurs spontaneously.