If a kid gets cancer at the age of 3 yrs. it is possible that his or her elder adult siblings can get cancer due to faulty genes or family history? if so, why the kid got cancer first while other siblings remained healthy till adulthood?

There are rare known family clusters of one or more cancer types & in those families age of onset of any can vary. The vast majority of childhood cancers at age 3 are not part of a family cluster/gene link. Gene or chromosome anomalies found in kids taners often occur as a new mutation. A consultation with a clinical geneticist can explore any family for such risk.