As some pointed out, those 3M+ it got from the sneak previews were at their vast majority from people who would have bought a ticket for OW, if not OD.
Obviously the presales are deflated but it's a matter of how much.Plus,WB has marketed this as a family movie, a genre that also thrives on walk ups and not presales .
And, yes, you can't expect this to blow up to something like 80M OW.It's an unknown character(and no previews exposition to a team up film) with a relatively unknown cast in a franchise that is trying to find its footing playing on a market that just a had a big superhero movies and with an even bigger coming next.