I have a current deck ,size 16' width x 12' out from the home, beams used were 8"x 12' on 16" centers. Supports used are 3- double 4x4 [4x8] 9' high from the ground.
Question, I plan to put a aluminum roof covering over the deck, will these support beams be adequate [3-double 4x4's] Thank You

I would take a close look at the following
The footers <concrete>at the bottom of the post
The condition of the post
What underneth
Reason I ask is you might want to go with some kind of re-inforcements to the post
There's many ways to strengthen them and still be decorative

You will need a permit for this and local codes as well as IBC will dictate much of what you can/can't do. I can not answer your question because I have no idea what your roof and trusses will weigh or what you have for post footings right now.