well i dont understand how tang tian and his friends go to Three Spirits City from where ever they are
Its just Tang Tian and his Spirit Generals that can. They go through their connection to the Southern Cross artifact which is inside Tang Tian. That's why no one else from the group can go back and forth except Tang Tian and his Spirit Generals, aka why in the last chapter Crane got a Weak Spirit General so he has a Liaison to Three Spirits City.
Think of Tang Tiang's artifact as a dedicated "hearthstone" to three Spirits City, apart from other functions. Others cannot use the pendant, but he and all spirit generals related to him can.