Yes, technically AM&tW won't be the least successful MCU sequel, but when you weigh in inflation, it will be by a pretty decent amount. Not that it matters as the film is in the green now and still hasn't opened in some markets like Japan and a few others.
I do think that the chances of an Ant-Man 3 are pretty slim though considering his arc is likely going to be fulfilled in Avengers 4. Plus Ant-Man 2 hasn't done as well worldwide as Ant-Man did. Even though I enjoy the films, if i'm Marvel, i'm probably pushing Ant-Man and Wasp into Hawkeye and Black Widow roles where they appear in other films, but are mainly supporting.
Especially with X-Men in play for Phase 4.