Here's a symbol we created by grouping three shapes together. Select the shapes you want to include in your symbol and choose Group Objects from the Group command on the Home tab.
Here's one we made by using the polygon line tool.
Next, you'll want to create a custom library and your new symbols to it.
Click on More in the symbols section of the SmartPanel on the left hand side of your workspace.
This will open the Symbol Libraries dialog.
Click on Add New and select Add New Custom Library.
Name your library and you'll have an empty library docked next to your drawing area.
To add your symbol, drag it from the drawing area over the library until you see the cursor change to the add symbol shape.
Now, you can name your symbol and choose whether you want it to change colors according to the diagram's theme or keep the colors you've designed it with.