ASUS India today has started rolling out Android Nougat FOTA update for Zenfone 3 (ZE552KL & ZE520KL) models. It was announced today on ZenTalk India (ASUS’s official Forum). This update will reflect to all the Indian users within this week. If not, then you can Manually search for update. (Go to Setting > About >System Update-to update manually).
To know detail about this update please check the source that has all the details to Get Android 7.0 in your Zenfone 3.