Your father was having sale deed in his favor. Then, how he has transferred in the name of your mother. It was through Will or by gift deed? Anyhow, in case your mother was the owner of the property by virtue of title documents, after her death, you are the owners of the property in equal share i.e. 1/3rd share each. There is no need to get any document registered in your favor. However, in case you want to partition the property, you can get the partition deed executed in your favor. Or you may gift your share to the other co owner or relinquish in favor of other co owner as the case may be. As such, if you want to remain co owners, you all three can sell the property on the basis of sale documents in favor of your parents accompanying with their death certificate.