Я бы просто использовал временный счетчик.
import re
afterlines=3
temporary_lines = ''
with open(filename, 'r') as f:
for line in f:
if line.strip() == ls_losses:
counter = afterlines
if counter > 0:
temporary_lines += f.readline()
counter -= 1
else:
print(temporary_lines)
temporary_lines = '' # Reinitialize to get ready for the next match
print ("true")