What's the best way to play against this line? (in the QGA)

TwoMove

It doesn't 3...e5 is the classic response to 3e4 too.  3.e3 e5 is by no means unusual and any decent book on the Queen's Gambit accepted. The article mentioned by IM Pfren is one of the best ways to learn white's ideas.

Optimissed

It's amazing how many people I play at Blitz around the 1700 rating mark try to hold the pawn. Most of the ones I lose, I lose on time. Maybe one in ten I blunder and get a bad position. Most of the time it's easy to counter by carefully selecting white's standard moves like a4, b3, Ne5, e4 etc.