This should have a pretty good multiplier for the weekend and good word of mouth. It's got an A cinemascore so the first Avengers still holds the top spot with an A+, but I think this will have very good legs.
I think people are getting too hung up over if this is an Avengers film, or if this is a Captain America film, the big issue for Marvel is the fact that with year after year release, it's hard to capture the same level of enthusiasm that they had when they came out with the first Avengers.
Overall this is a very high praise for this film, and it's a good number, will probably be in the IM3 range, hopefully better, maybe 180-185 with a big multiplier, but I was always cautious of thinking this would do over 200M.
I did not see the film in 3D but from what I've heard the 3D wasn't that good, and I can understand why with a lot of shaky cam action that just doesn't translate well in 3D. I have a feeling there will be a lower level of 3D compared to either Avengers film and that is going to hold it's totals back.