The big pressure thief is likely the 3/8" line, especially if they are long (longer than 50 foot each, thereabouts). Going to 5/8" line will make a nice difference with just the 2 diffusers. The Gast 0523 can certainly run 3 diffusers so long as they are no deeper than 10 foot each and have 5/8" ID line. That's my 0523 setup...5/8" line, one at 70/80'. the other 2 at 50/60 foot, and the deepest at 7'. It runs at 5-6 psi. Your not at a pump "kill" scenario at 10 psi, but you could do better and add some life to the pump. I would not stress about it, but adding 5/8" line would reduce your back pressure and, likely, increase the overall airflow.

Fish on!,
Noel