You can do one thing 

immediately file a suit for declaration as legal heir and claim 1/3 rd share of the proceeds received for the property sold. 

also you can file an application for revocation of letters of administration and if there are other properties then you can easily claim. 

they ought o have given notice to you if you are a direct legal heir as per the schedule to Succession Act

The letter of administration obtained by concealing facts, then such certif8cate can be set aside through court by filing necessary application in court. Thereafter  sake made by such administrator in garb of letter of administration should also be challenged and be declared null and void. However, purchaser being bona-fide purchaser has independent right. 

Yes, the Letter of Administration (LOA) can be revoked if there is a third person, who is a genuine and bonafide legal heir, puts his claim on the property. This is because the LOA is only valid if it is granted to all the legal heirs of the deceased. If any legal heir is not included in the LOA, then the LOA can be revoked by the court.

In the case you have mentioned, if the third person is a genuine and bonafide legal heir, then he/she can file an application in the court to revoke the LOA granted to A and B. The court will then consider the application and decide whether to revoke the LOA or not.

If the LOA is revoked, then the property will be vested in the third person as a legal heir. The purchaser of the property will not be entitled to the property.

The Supreme Court has held that Letters of Administration can be revoked on the ground that all the legal heirs were not impleaded in the petition under Section 276 of the Indian Succession Act for the grant of Letters of Administration with Will annexed.

However an applicant in order to succeed in revoking the letters of Administration granted by the court must establish just cause within the meaning of Section 263 of the Indian Succession Act, as laid down in clauses (a) and (e) thereof. 

The enumeration of circumstances in the explanation to Section 263 of the Act is exhaustive and not merely illustrative. The circumstances under which a grant can be revoked under this Section may be classified under the following heads for easy reference:-

(i)When there is substantial defect in the proceedings to obtain the grant.

(ii)When the grant is obtained under false suggestion.

(iii)When a subsequent Will or codicil is discovered; 

(iv)When required for the better Administration of the estate; 

(v)When the person to whom the grant was made willfully neglects to file an inventory and accounts

The aggrieved person can challenge the grant of letters of administration in the appropriate court.

The property was sold fraudulently by excluding one of the legal heirs on the basis of letters of administration obtained by playing fraud on court.

The aggrieved legal heir can file a suit for partition and separate possession of his share in the property.

If the property purchased  by the buyer falls within the share of property to be allotted to the aggrieved legal heir then the buyer will be in trouble.

The buyer has to lament over his mistake of not obtaining proper legal opinion before venturing into the purchase of this property.

Yes it will be revoked and will be treated as fraud played upon the court by not providing the true facts and information about all legal heirs on record 

LOA can be revoked if notice was not issued to all legal heirs 

you have to file suit to set aside sale of property 

seek injunction restraining purchaser from selling the property pending hearing and final disposal of suit 

Court would set aside fraudulent sale as legal heir was not issued any notice and LA obtained by suppression of material facts 

- The Supreme Court held that, a Letters of Administration can be revoked on the ground that all the legal heirs were not impleaded in the petition under Section 276 of the Indian Succession Act for the grant of Letters of Administration with Will annexed.

- Hence , you can file a suit for declaration before the court for declaring the sale deed in favour of B invalid after cancelling the letter of administration.

If a letter of administration has been obtained through the concealment of facts, it is possible to have this certificate invalidated through a court application. Subsequently, any transactions carried out by the administrator under the guise of the letter of administration can also be contested and declared invalid. It is important to note that a purchaser who acted in good faith maintains an independent legal right in such circumstances.

you need to tread with great caution 

there is a saying that if things are on one's side then even an elephant can pass through a needle hole and if the things are not working then even an ant can get trapped in that hole

suppressing the fact that there is a 3rd heir of the deceased is firstly a fraud exercised on the Court

the 3rd heir can at anytime when he learns that a LA was obtained without citing him, can apply for revocation of that LA and all the acts done by the LA holders so far

if the buyer of the property is also aware about there being a 3rd heir and despite that he proceeds with the transaction then he cannot be said to be a 'bonafide purchaser for value without notice'. so even such buyer would be a party to the fraud committed by A & B

Mere issuing public notice in newspapers and absence of any objections to such notice by any person does not absolve A& B or their buyer with the legal consequences when the fact regarding existence of a 3rd heir comes to the surface

such buyer would also be defrauding the lender bank which would be sanctioning the loan to finance the purchase of the property from A&B

A&B as LOA holders are also legally required to file an inventory report in the Court as to how and in what manner they dealt with the assets of the deceased