Need to add hours min and seconds

[tester_V]

Hi,
I have a problem adding HHMMSS.
Have a file, it goes like this:

5/1/2020 2:40:31 PM, 5/1/2020 2:43:11 PM, 00:02:40.5449136, PV0015EN,Cell_13, TT
5/1/2020 2:45:26 PM, 5/1/2020 2:48:17 PM, 00:02:50.8250927, PV0015EN,Cell_13, TT
5/1/2020 2:53:18 PM, 5/1/2020 2:56:01 PM, 00:02:43.7207947, PV0015EN,Cell_13, TT
5/1/2020 3:04:43 PM, 5/1/2020 3:05:42 PM, 00:00:59.2069877, PV0015EN,Cell_13, TT
5/1/2020 2:53:18 PM, 5/1/2020 2:56:01 PM, 00:02:43.7207947, PV0015EN,Cell_13, TT
5/1/2020 3:04:43 PM, 5/1/2020 3:05:42 PM, 00:00:59.2069877, PV0015EN,Cell_13, TT

The third column is a time duration and I need to sum it up.
I thought I will split each line, get the third column (timeduration) then will split it (will remove milliseconds) and add separately Minutes and Seconds.
And after that will convert using something like this:

sec= 66
fg= str(datetime.timedelta(seconds=sec))
print (fg)

min= 25
fm=str(datetime.timedelta(minutes=min))
print (fm)

I created little script but stuck - do not know how to add my vars:
Her is my code:

import os
too = 'C:/01/OUTCOME.txt'

with open(too) as fo:        
            for line in fo:
                tu, on,ontu,*stu= line.split(',')
                ontu=ontu.rstrip()
                sd,st=ontu.split('.')
                #print (sd)
                sdh,sdm,sds=sd.split(':')
                print ("Minutes", sdm)
                print ("Seconds", sds)

Thanks in advance!