如何连接不同N行的Pandas数据帧？

import pandas as pd

df1 = pd.DataFrame({'A': [1, 2, 3], 'B': [4, 5, 6]})
df2 = pd.DataFrame({'A': [7, 8, 9], 'B': [10, 11, 12]})
df3 = pd.concat([df1, df2], axis=0)

import pandas as pd

df1 = pd.DataFrame({'A': [1, 2, 3], 'B': [4, 5, 6]})
df2 = pd.DataFrame({'C': [7, 8, 9], 'D': [10, 11, 12]})
df3 = pd.concat([df1, df2], axis=1)

import pandas as pd

df1 = pd.DataFrame({'A': [1, 2, 3], 'B': [4, 5, 6]}, index=[0, 1, 2])
df2 = pd.DataFrame({'A': [7, 8, 9], 'B': [10, 11, 12]}, index=[3, 4, 5])
df3 = pd.concat([df1, df2], ignore_index=True)