Taskbar/system tray suddenly changed to 3 rows. How do I get my one row taskbar back in Windows 7?

The system tray does only give more information (on the clock etc) because it has the space to do so. Please understand that the system tray is part of the taskbar and as such will resize to whatever size the taskbar is.

What you need to do is ensure that the taskbar is unlocked (right-click and make sure there is no checkbox in front of "lock the taskbar". If so, click it, to remove the checkbox.

Now, hover the mouse to the edge of the taskbar that is not touching the edge of the screen (the taskbar touches the edge of the screen on 3 sides, and the desktop on 1 side, you want to find this last edge.)

The mouse cursor will change to a resize icon. Now drag and hold the left mouse button and move the mouse as far to the edge of the screen as possible, making it smaller. It will move in intervals because the taskbar has fixed widths. Just move it as far as you can until the taskbar has the desired size.

For the best results, drag the taskbar in its center.

You may need to reorder certain taskbar elements. You can do this by dragging the | beam at the very left of each taskbar section (if applicable)

Once done, lock the task bar again by right-clicking the bar and select Lock the taskbar.