df = df[(df.risk.isin(["Small","Medium","High"]))]

例：

In [5]:
import pandas as pd
df = pd.DataFrame({'risk':['Small','High','Medium','Negligible','Very High']})
df
Out[5]:
         risk
0       Small
1        High
2      Medium
3  Negligible
4   Very High
[5 rows x 1 columns]
In [6]:
df[df.risk.isin(['Small','High'])]
Out[6]:
     risk
0   Small
1    High
2  Medium
[3 rows x 1 columns]