To apply a function over multiple columns in a DataFrame, the list of columns can be iterated over using a for loop. In the following example, a predefined list of columns is converted from the string type to the numeric type:

df['new_col5'] = ['7', '8', '9']
df['new_col6'] = ['10', '11', '12']

for str_col in ['new_col5','new_col6']:
    df[[str_col]] = df[[str_col]].apply(pd.to_numeric)
    
print(df)

Here is the output:

  col3 new_col1  new_col2  new_col3  new_col4  new_col5  new_col6
0    x                  0         5         5         7        10
1    y                  0         7         7         8        11
2    z                  0         9         9         9        12