Why do young 3-year-old children hate their young siblings?

 This is usually common in children of the same sex. It may end with age and parents' counselling of the concerned child, but at times the traces of this kind of behaviour continue to be manifest even at later stages of development. But How does it come about?

Social development in children involves pre-schoolers interactions with other people. They continue to show attachment to their caregivers but at age 3, they are less distressed when separated from them. 

At age 4, children have less of the need to be near the caregivers since their egocentricity is declining. However, at the age of three, with the siblings, children become jealous of a new baby since they have to share their parents’ attention and affection, the phenomenon commonly known as Cain Complex. It is displayed as a brusque latent or manifested rivalry between brothers or/and sisters for achieving all parental love and it is sometimes so destructive that it may lead to fratricide. 

In adulthood, a similar rivalry is also observed but the completion is then about achieving the highest social status Competition may take violent forms and accepts victory (supremacy) only of the one.

The term has Biblical Origins as from Cain who was the first son of Adam & Eve (Bible - Gen. 4.) and his offering to God was "of the fruit of the ground. Cain murdered his brother Abel whose offering to God was of the "firstlings of his flock and of the fat, out of jealousy. The 2 brothers, on one Sabbath, presented their offerings to the Lord. Abel's sacrifice was considered "more excellent" (Heb. 11:4) than Cain's, & was accepted by God. From this event Cain was "very wroth," & cherished feelings of murderous hatred against his brother; leading to the murder of Abel (1 John 3:12). For this crime, Cain was expelled from Eden and henceforth was condemned to be a fugitive.

In childhood, this feeling of jealousy does not last longer. With time, the siblings will be kissing each other, and then the relationships become ambivalent that is, they are friends at one moment and enemies at another moment. The birth order of siblings determines also their social interaction and relationships to some extent.