import re

my_string = "T-46-5-В,Г,6-В,Г"
my_pattern = r"^(\D-\d{1,2}-)[\d,]+(-[а-яёА-ЯЁ,]+,)[\d,]+(-[а-яёА-ЯЁ,]+)$"

new_string_parts = ["005", "006"]
new_string = re.sub(my_pattern, fr"\g<1>{new_string_parts[0]}\g<2>{new_string_parts[1]}\3", my_string)
print(new_string)
# =>   T-46-005-В,Г,006-В,Г