Unfortunately there are no definite answers to what is OK and what isn't. Its not like two 90 corners is totally fine, but three will completely ruin your performance.
In my opinion three 90 degree bends is probably going to be fine. I'd do as much as possible to allow the air to flow smoothly though. This means putting as large as radius as practical on the inside corners of the port, and putting 45 degree angled pieces on the outside corners. At the ends of your port I'd put as big of a flare as practical as well. Here is an example:
Have you modeled your sub in something like WinISD to see what port velocities you are going to have? The lower your port velocities are the more complex port you can get away with.
At 56" your port is getting to be pretty long, I hope you are planning on crossing over to this sub pretty low or you could have port resonance issues.