Is it permissible to travel a distance of 800 km to visit the grave of a deceased wife, and does this fall under the prohibition of "do not set out on a journey except to three mosques"?

Traveling to visit the grave of a deceased person is not permissible. It is advisable to refrain from visiting your wife's grave and instead do what benefits her in her grave, such as supplicating for her with mercy and forgiveness, and giving charity on her behalf with the intention of dedicating the reward.