Reddit: Low hanging jokes are easy to pump out and do very well. I have several 100K+ comment karma accounts. Find some rising threads where the kitchen sink could contribute and make a short (1 line) joke. For thread karma, simply repost to the default subreddits whatever was posted ~ a year ago.
Alternatively, you could copy and paste the top imgur comment, or the top Reddit comment if it is a repost. This would be easy to write a bot for.
HN: Jokes don't do that well here, for better in my opinion. Volume is much more important. Many 1-2 paragraph posts will do well for you, though this requires a lot of domain knowledge. Going after the low hanging fruit, like "hating" on Uber, or privacy qualms on Facebook, also helps as these tend to attract the most comments.
And of course, for both sites, going for breaking news topics when they are relatively fresh is a huge multiplier. Being 1h earlier is the difference between 10 points on HN and 100.
Stack Overflow: SO is very different. Aim to answer introductory level questions with a substantial amount of detail and links. Quality is much more than quantity. The key is answering homework questions that will be googled by undergrads (like myself, sorry) and viewed hundreds of thousands of times.