You have a few options. It is difficult to answer this question in isolation. The best solution may depend on other factors beyond this one joint. For example, if the 6 x 6 post were to extend up through the deck and support a railing, we would suggest hanging the built-up wooden beams off the post with joists hangers. Very often, the built-up wooden beams are woven together as opposed to having all six pieces of lumber ending over a 6 x 6 post. If for example one of the middle 2 x 10s was 3 feet longer, and the corresponding middle 2 x 10 was 3 feet shorter, it would be much easier to tie the two beams together. If the lumber is already cut, you could lag a 4 foot piece of 2 x 10 on to the interior, where it won't be seen, to tie the pieces together. In all cases, once the joists are fastened to the beams, the tops of the beams will be laterally supported which will eliminate their ability to rotate.