What three can do that two cannot
A diatomic molecule can oscillate in only one way. Molecules of three or more atoms have more possibilities. The above applet allows you to visualize the three different patterns of vibration (‘modes’) which a triatomic molecule like CO2 can exhbit. There will be a little more about ‘modes’ later in the course; and lots more in subsequent courses.
You might be puzzled by the fact that the applet quotes the frequencies in units of cm-1. This is a convention favoured by spectroscopists (especially if they are chemists). To convert to frequencies in Hz you must multiply the numbers given by the velocity of light, c, expressed in cm⋅s-1.
http://chemmac1.usc.edu/bruno/java/Vibrate.html