There is little to be added to Garrett's great answer, but pandas also has a scatter
method. Using that, it's as easy as
df = pd.DataFrame(np.random.randn(10,2), columns=['col1','col2'])
df['col3'] = np.arange(len(df))**2 * 100 + 100
df.plot.scatter('col1', 'col2', df['col3'])