pandas replace multiple values one column

Your replace format is off

In [21]: df = pd.DataFrame({'a':['Small', 'Medium', 'High']})

In [22]: df
Out[22]: 
        a
0   Small
1  Medium
2    High

[3 rows x 1 columns]

In [23]: df.replace({'a' : { 'Medium' : 2, 'Small' : 1, 'High' : 3 }})
Out[23]: 
   a
0  1
1  2
2  3

[3 rows x 1 columns]