To merge three FFT spectrum graphs into one figure, start by opening a new figure in your programming environment. Use the "figure" command to create an empty figure window. Access the Figure Palette to create three 3D axes for your graphs. Right-click on each axis to add your data for the three spectra, labeled as Z1, Z2, and Z3. To automate this process in code, utilize the subplot command to divide the figure into three sections, allowing for the sequential display of each graph. This method ensures a clear and organized presentation of all three FFT spectra in a single visual output.