بسم الله الرحمن الرحيم
There could be maximum seven shares in a big animal. However, less than seven people may also sacrifice a big animal together but it is mandatory that the amount of any one is not less than seventh share. If he gives amount equal to seventh share then he will be eligible for the meat of seventh share. Thus in the question mentioned above a father and two of his sons may sacrifice a big animal together and all the trio should pay equal amount, it is lawful.
Allah knows Best!
Darul Uloom Deoband
This answer was collected from the official ifta website of Darul Uloom Deoband in India.