Using timedeltas (tested in Python 3.4):
import datetime
timeList = ['0:00:00', '0:00:15', '9:30:56']
sum = datetime.timedelta()
for i in timeList:
(h, m, s) = i.split(':')
d = datetime.timedelta(hours=int(h), minutes=int(m), seconds=int(s))
sum += d
print(str(sum))
Result:
9:31:11