Varanasi is one of the oldest continuously inhabited cities in the world, and it does not hurry for visitors. People come to bathe, to pray, to cremate their dead and to be cremated themselves, and all of it happens in the open along the riverbank. It can be moving, confusing and exhausting in the same hour.
Three days is the right amount. You arrive and see the evening aarti, spend a full morning on the river and in the lanes, visit Sarnath in the afternoon, and still have time to rest. Your guide explains what you are seeing, where to stand and what not to photograph, so you can be present instead of worried.
If you can give a fourth day, we can add a quieter second morning, silk weaving, or a day trip.