Hi,
To convert multiple columns to string, include a list of columns to your above-mentioned command:
df[['one', 'two', 'three']] = df[['one', 'two', 'three']].astype(str)
# you can add any number of columns
To convert all columns into string, you need to construct the list of columns:
all_columns = list(df) # Creates list of all column headers
df[all_columns] = df[all_columns].astype(str)