Oh! I had the idea that new posts start with 3 karma, but then I was wrong.
They start with a self-strong-upvote, which depends on the user karma https://​​github.com/​​centre-for-effective-altruism/​​ea-forum/​​blob/​​27d24f53b7dce59b770040e6ac9ff05fcb9c4b16/​​src/​​lib/​​votes/​​voteHelpers.ts#L71-L91
They start with a self-strong-upvote, which depends on the user karma https://​​github.com/​​centre-for-effective-altruism/​​ea-forum/​​blob/​​27d24f53b7dce59b770040e6ac9ff05fcb9c4b16/​​src/​​lib/​​votes/​​voteHelpers.ts#L71-L91