place the symbol

to add certain symbol after every three got splited lines Something like that?? –

print('@'.join(st_text[i:i + 3] for i in range(0, len(st_tex), 3)))

Answer

This code looks if there are three or more commas in the text. If it is, it splits the text into 4 parts, then concats the 3 first parts and adds an @ before the third comma and redo the same process with the remaining text (4th part):

text = "Reading practice to help you understand simple texts and find specific information in everyday material. Texts include emails, invitations, personal messages, tips, notices and signs. Texts include articles, reports, messages, short stories and reviews."
result = ''
while len(splitted := text.split(',', 3)) == 4:
    result += splitted[0] + ',' + splitted[1] + ',' + splitted[2] + '@,'
    text = splitted[3]
result += text

result == "Reading practice to help you understand simple texts and find specific information in everyday material. Texts include emails, invitations, personal messages@, tips, notices and signs. Texts include articles, reports@, messages, short stories and reviews."
>> True