Is it permissible to give a 3-kilogram bag of rice to two people who will share it, as part of an expiation for an oath, and is it sufficient to give a quarter or a half of a chicken with rice from a restaurant as Zakat?

What you did, giving each of the ten needy people one and a half kilograms, suffices as expiation for the oath. The expiation for an oath is fulfilled by feeding the needy a lunch and a dinner meal from the average of what you feed your family, whether it's a quarter chicken with rice or other food, in a restaurant or at home. A single meal is not sufficient.