Can we construct another floor if ground floor owner doesn't agree?

Yes we can do so but it depends on certain factors. In some cases, permission is required. Depends from case to case

If the owner of the ground floor is not you then you cannot construct the second ,third floor.And if you are staying in the second floor and intend to construct the third floor,you need to check whether the FSI of the plot permits you to build it with the permission of the ground floor.Provide full details of the owner of the ground floor and your title in the plot .