One of the legal heirs died, what will happen now

Yes. The property shall be in three equal shares among you, your mother, and your brother.

Some where I read it will be shared with legal heirs of my grandmother can you please clarify that?

In your father's property, only your grandmother if alive at that time has a share.  But now she is no more no others have right.

Thanks sir for the reply

Technically, legal heirs of your grandmother have rights devolved upon them unless  Section 15 of the Hindu Succession Act has been amended as in Kerala.

Any property inherited by a female Hindu from her pre-deceased son shall devolve, not upon the other heirs referred to in sub-section (1) in the order specified therein, but upon the heirs of the pre-deceased son from she inherited the property.

Whether your grandmother has executed any will or died intestate ?

If she has died intestate her legal representatives shall succeed in her share of the property.

It is better to consult a local prudent lawyer for analyses of facts/ documents and professional advise.

I will go with the opinon given by Mr.P.Venu and Dr.JC Vashista

If the proeprty was not partitioned after the death of the property owner by the legal heirs and have not taken physical possession  of their respective shares in the proeprty then it can be construed that the deceased grandmother did not inherit her share of property. 

Therefore upon her death, her share of property shall revert on the other legal heirs of her  deceased son.

Thus the proeprty can be divided into three shares among the existing class I legal heirs of the deceased. 

In India intestate succession of property depends on what religion does each person follows and according to that personal laws will apply. You would fall under the Hindu Succession Act 1956. Section 8 to 13 of the Hindu Succession Act 1956 deals with the case of intestate succession. Section 8 of the Act deals with what happens if Hindu male dies intestate. It states that if such Hindu male dies intestate the property shall go to class 1 heirs and if there are no class 1 heirs then only class 2 heirs will acquire interest in such property. As per your case the father had died leaving your grandmother, your brother, mother, and you. So according to this case mother, your brother and you belong to class 1 heirs which is dealt with under Hindu Succession Act 1956. While your grandmother falls in class 2 heir subcategory 5. And hence my understanding is that the grandmother will not get any share in the property whereas the mother, brother and you will divide according to Section 10 of the Act.