What is stronger a 6 x 6 post or 3 2 x 6 bolted together?

It kind of depends on what you are doing and how you want the finished project to look. As a post with the weight or pressure straight down, it really doesn't make a lot of difference. Any side stress, 3 bolted together would always have more strength because of the different grains in the wood going in different directions. A single solid post can snap if there happens to be a flaw that runs all the way through it.