On GPT-3's large number addition, a little prompt engineering goes a long ways. Separating large numbers into three digit chunks is helpful for its input encoding (https://www.gwern.net/GPT-3#bpes), and giving it a few examples of other correct large sums will make the correct answer the most likely continuation of the input.

For example, I was able to get 2/3 nine digit sums correct (the third one was off by exactly 1000, which is interesting) by using this prompt:

    Here are some examples of adding large numbers:
      606 468 720 + 217 363 426 = 823 832 146
      930 867 960 + 477 524 122 = 1 408 392 082
      823 165 449 + 493 959 765 = 1 317 125 214

And then posing the actual problem as a new line formatted the same up to the equals.