Bracket to lay 3× 2x6 on top a 4x4 post?
You want the post and beam to have the same width dimension. Although you're trying to recycle, it's best to build this with a 6x6 beam and 6x6 posts. It will be stronger and look nicer anyways. Attach them together with the Simpson LCE4 seen below. A total of 4 of them will be needed and each of them will need (14) 0.162 x 3 1/2 into the beam and (10) 0.162 x 3 1/2 into the post.