Is it obligatory to return the money to the owner of a laptop that was sold to him with an undisclosed defect, knowing that three years have passed since then, and is it permissible to give the money in charity instead of returning it, for fear of the buyer's reaction?

You should have informed the buyer about the defect in the computer, as concealing it is a forbidden deception. The sale is valid, but the buyer has the option to either annul it or keep it. If the intended benefits of the device are lost due to damage, you must pay compensation (arsh) to the buyer. Giving charity with the money is not sufficient, and you must deliver it to him without informing him of the reason if you fear his reaction.