Extract substring from dataframe and add it to new column.

How to extract a substring from column and add it back as new column.

For example, I have file1 with 2 column. I want extract first 3 character from fullname column and then I need to add it as new column.

File1
FullName|count
APPLE|100
ORANG|200
GRAPES|300

I need below output
Output1
FullName|ShortName|count
APPLE |APP|100
ORANGE |ORA|200
GRAPES|GRA|300

Is below is correct way of doing this.
Script
df = pd.read_csv('File1', sep='|')
df['ShortName'] = df['FullName'].str.slice[0:3]
df.out_csv('output1')