Do a search on "GM-LL" & you'll find some previous info. Also, the VOA section has results from both of Saab's current OE oils & the M1 0W-40. I can tell you what I know from my research into this spec, but since GM refused to send me any info directly, some of this is deduction on my part.
In a nutshell, no, ACEA A3 is not the same thing. This new GM spec appears to have a fuel-efficiency aspect to it, so most A3 oils are too viscous to pass this portion. This doesn't mean A3 oils are unsatisfactory; in fact, M1 15W-50 will protect as well or better in your turbo engine than either the Saab 0W-30 or M1 0W-40. And Delvac 1 5W-40 combines the protective film of the M1 15W-50 with the additive package of the OE 0W-30. (But you wouldn't want to deal with the hassle of trying to educate your dealer or Saab Customer Assistance about it!)
Basically, it's requirement is an attempt to do two things:
1) Mitigate the problems created by using sub-premium oil in an oil-killing engine; part of the solution was to require only premium, fully synthetic oil via the spec, while the 50% increase in oil capacity & addition of GM's Oil Life Monitor handles the rest.
2) Earn GM more environmental credit $ by further reducing fluid waste products & increasing fuel efficiency.
In my opinion, M1 0W-40 should be the thinnest oil you should use in a Saab turbo engine.