I get TLE with the following solution on codechef, I loop through only once keep two lists and find the majority

can someone help me to understand what should I do to not exceed the time limit?

l1, l2, l3 = tuple(map(int, input().split()))
 
final = []
interm = []
i = 0
while i< l1+l2+l3:
    id_num = input().strip()
    if id_num not in interm:
        interm.append(id_num)
    elif id_num in interm and id_num not in final:
        final.append(id_num)
    i+=1
 
final.sort()
print(len(final))
print('\n'.join(final))