Advent of code - Day 3 - Second task

alphabet = "abcdefghijklmnopqrstuvwxyzABCDEFGHIJKLMNOPQRSTUVWXYZ"
duplicate_chars = []
total_sum = 0
counter = 1

with open("text", "r") as input_data:
    for line in input_data:
        if counter == 1:
            first_line = line
        if counter == 2:
            second_line = line
        if counter == 3:
            temporary_chars = []
            third_line = line
            for ch in first_line:
                if ch in second_line and ch in third_line and ch not in temporary_chars and ch != "\n":
                    temporary_chars.append(ch)
            duplicate_chars.extend(temporary_chars)
            counter = 0
        counter += 1

for ch in duplicate_chars:
    total_sum += alphabet.index(ch) + 1

print(total_sum)