I think it is only certain species of Cuckoos which eject the eggs from the nest. These include Common Cuckoo, which is the best studied species. One reason is that Common Cuckoo is considerably larger than the host, so needs all the food that the parent can collect.
Other species don't have this size difference. Some species may lay more than one egg in the same nest, and some species are raised with the chicks of the host. Koel is probably one of these species.
I would think it is likely that these three are from the same nest, and they are probably brothers. It is interesting that it appears to be three males.
