for index1, ls in enumerate(myLists):
    for index2, x in enumerate(ls):
        merged_list.append((x, index1, index2))
merged_list.sort()
st = set()
res = [(-1) for i in range(len(myLists))]
for x, y, z in merged_list:
    if res[y] != -1 or z in st:
        continue
res[y] = z
st.add(z)
print(res)
Getting unique indices of minimum values in multiple lists