Purchase Of Property

Wants to buy property, but the seller demands 70% of the total consideration of the property. Remaining during sale deed after 3 months. 3 months' time taken is due to one of the heir out of 3 (this is a mother and 2 sons) will complete 18 years age by Feb 2021. Ready to give possession and original Doc's during sale agreement stage itself. Property is freehold. Purchase is outright. Your insights on legal guards. If the sale agreement is registered, does it have legal validity as 2 will only sign ( mother and one son)