timeList = [ '0:00:00', '0:00:15', '9:30:56' ]
totalSecs = 0
for tm in timeList:
    timeParts = [int(s) for s in tm.split(':')]
    totalSecs += (timeParts[0] * 60 + timeParts[1]) * 60 + timeParts[2]
totalSecs, sec = divmod(totalSecs, 60)
hr, min = divmod(totalSecs, 60)
print "%d:%02d:%02d" % (hr, min, sec)