The easiest way would be to determine which one of the profiles will be your main profile.

Open one of the other profiles and add the songs to a schedule and save the schedule. Then open the main profile and open that schedule and import the songs from the schedule to the database. For the media, you can right click on any piece of media and select Open file location. That will open the location of your media. You can select all the videos from that profile and copy/cut and paste them into the videos folder in your main profile folder and then do the same for Images and Audio.
Then, do the same thing for the other profile you want to merge into the main profile.