Use .mean. By specifying the axis you can take the average across the row or the column.
df['average'] = df.mean(axis=1)
df
returns
salary_1 salary_2 salary_3 average
0 230 235 210 225.000000
1 345 375 385 368.333333
2 222 292 260 258.000000
If you only want the mean of a few you can select only those columns. E.g.
df['average_1_3'] = df[['salary_1', 'salary_3']].mean(axis=1)
df
returns
salary_1 salary_2 salary_3 average_1_3
0 230 235 210 220.0
1 345 375 385 365.0
2 222 292 260 241.0