can an insurance company deny a fire claim when the investigation done by police, fire and 3rd party cant find the cause?

It would be very difficult for an insurance company to deny a fire claim if there is no evidence of how the fire started.  They cannot assume that the cause was arson, or intentional if there is no such evidence.

You should contact your insurance agent to see if they can help you get the company to move forward on this claim.  You can also call the insurance company and speak to a supervisor to try to get the claim paid.  Anything you have from the police and fire that indicates they have no evidence of an intentional fire should be provided to the insurance company to strengthen your claim.  You should also ask for the reasoning, in writing, as to why the company is denying the claim.