>>> l1 = ["1" ,"2"]
>>> l2 = "3;4"
>>> l3 = ["5", "6"]
>>>
>>> for x in [l1, l2, l3]:
    print("Working on: {}".format(x))
    if isinstance(x, basestring):
        x = x.split(";")
        print("New x: {}".format(x))
    else:
        print("Its same")


Working on: ['1', '2']
Its same
Working on: 3;4
New x: ['3', '4']
Working on: ['5', '6']
Its same
>>> l1, l2, l3
(['1', '2'], '3;4', ['5', '6'])

>>> l1, l2, l3
(['1', '2'], ['3', '4'], ['5', '6'])

x = x.split(";")

def splitter(x):
    print("Working on: {}".format(x))
    if isinstance(x, basestring):
        x = x.split(";")
        print("New x: {}".format(x))
    else:
        print("It's the same")
    return x

l1 = splitter(l1)
l2 = splitter(l2)
l3 = splitter(l3)

l1, l2, l3 = [splitter(x) for x in [l1, l2, l3]]

vals = [l1, l2, l3] # create list
for i, x in enumerate(vals): # iterate through list
    vals[i] = splitter(x) # modify list
l1, l2, l3 = vals # unpack list

l1,l2,l3 = [i.split(';') if isinstance(i, basestring) else i for i in [l1,l2,l3]]

>>> print l1, l2, l3
['1', '2'], ['3', '4'], ['5', '6']