Для разбора времени используйте strptime
>>> x = '00:03:12,200 --> 00:03:14,316'
>>> x1 = x.split('-->')
>>> x1
['00:03:12,200 ', ' 00:03:14,316']
>>> t1 = datetime.datetime.strptime(x1[0], '%H:%M:%S,%f ')
>>> t1
datetime.datetime(1900, 1, 1, 0, 3, 12, 200000)
используйте timedelta
>>> t = datetime.timedelta(seconds=1)
>>> t
datetime.timedelta(0, 1)
>>>
>>> t1 + t
datetime.datetime(1900, 1, 1, 0, 3, 13, 200000)
>>> k = t1 + t
Reconvert to string
>>> k.strftime('%H:%M:%S,%f ')
'00:03:13,200000 '