You can use the integers by calling their position in the list. Here’s an example of adding, subtracting, and finding the sum of your list:
example = [12, 3, 4]
print(example[0] + example[1]) #15 (12 + 3)
print(example[2] - example[1]) #1 (4 - 3)
print(sum(example)) # 19 (12 + 3 + 4)
solved Python: How can I use a specific integer from a list?