I have a 17.5 by 8 2nd floor deck there are 3 posts for support and I want to go to to the deck is supported by 2 buy 8 boards I don't want it to sag I'm going with 6*6 posts and want to stay with the same 2*8 mat. What do you think to stop any sagging

Your question is unclear. "I want to go to to the deck is .....by 2 buy 8"? Photo or drawing may help as we find it hard to determine exactly what you are trying to accomplish.

There are national and local building codes for decks. The dimensions and posts are just 2 parts of it. The distance between the joists matters, as do beams or girders dimensions. So yes, you need the entire plan.
And if you do this without a permit and are seen, you may find that it all has to come down to do the footings and piers to current code.