The penalties for faking a signature include criminal charges, jail time, documents annulment, money reimbursement for the victim, or more. If you are a victim of signature forgery, it is highly recommended to seek legal advice from an attorney immediately as fraud or forgery cases often have a statute of limitations.
What you can do is file an FIR against him in relation to forgery, under sec 420 and 406 of the IPC, and while doing so it will be convenient for you if you will take along with you some documents showing that the sign has been forged.
Consult a good lawyer with your details.
The opinion of a handwriting expert is secondary evidence under the provisions of sec 45 of the Indian evidence act and so the court may or may not rely on the opinion of a handwriting expert as it is not conclusive proof of evidence therefore what you can do is use the expert opinion for cross-examination of witnesses who signed the third document demolish the fact that he had signed the third document.