You can make use of the Audacity application to join audio files. Audacity is free to use, so it shouldn't be a problem in terms of registration or anything related to this.
Download the application and install it. Open Audacity from the icon on your desktop and select Open then choose your file. It will be opened in full wave-form mode. Press CTRL+N and open another file and you can do that for the remaining one. Now, leave the first window opened and select the second one. Select the whole track by pressing CTRL+A or using your mouse to drag from one side to another and press CTRL+C; then switch to the first window and move the marker at the end of the track. Press CTRL+V to paste the second file at the end of the first one. Do the same with the third audio file and in the main window go to File > Export. Choose the output format, name, codec, etc. and that's it. You will have one file which contains all the parts.