How do I install a new header in an exterior load-bearing wall?
Assuming that the 3 2x8s will work for you - and it sounds right for your situation then you are fine with adding some sort of plywood to make your header flush. 

However I usually (given I have the space) would throw down a 2x6 flat on top of the 2x8s that are on the jacks (yes you must have jacks for your header).  This gives good reference points for top and bottom.