import itertools

somelists = [
    ["Z", "A", "Y"],
    ["X", "2", "W"],
    ["V", "a", "U"]
]

first = ('A', '2', 'a')
first_index, prev_len = 0, 1
for sub_list, sub_first in zip(reversed(somelists), reversed(first)):
    first_index += sub_list.index(sub_first) * prev_len
    prev_len = len(sub_list)

for element in itertools.islice(itertools.product(*somelists), first_index, None):
    print(''.join(element))
    break

#expected output: A2a
#actual output: ZWa