1. A medical diagnosis apparatus comprising: a scanner configured to scan a three-dimensional region including a target region and acquire data of the three-dimensional region; and processing circuitry configured to: generate a first three-dimensional medical image based on the data; discretely arrange fixed points framing a first region of interest (ROI) corresponding to the target region in the first three-dimensional medical image; set apexes of a second ROI based on some fixed points included in the fixed points; receive an instruction from an input device used by a user; and change, in accordance with the received instruction, at least one of the apexes based on at least one of the fixed points other than the some fixed points while maintaining positions of the fixed points.