1. A method for combining three-dimensional ultrasound image data, comprising: obtaining a plurality of three-dimensional ultrasound image data that is associated with a Region of Interest (ROI); detecting one or more landmarks in each of the plurality of three-dimensional ultrasound image data using a parameter associated with the one or more landmarks, the one or more landmarks comprising a feature distinct from surroundings of the one or more landmarks; outputting the detected one or more landmarks; receiving a user input, the user input selecting at least one landmark among the detected one or more landmarks or changing the parameter associated with the one or more landmark according to a number of the detected one or more landmarks; in response to receiving the user input changing the parameter, updating the parameter based on a changed parameter, and repeating the detecting, the outputting, and the receiving based on the updated parameter; and in response to receiving the user input selecting the at least one landmark, generating a combined three dimensional ultrasound image, by combining two pieces of the plurality of three-dimensional ultrasound image data using at least one landmark which is commonly included in the two pieces among the selected at least one landmark.