Disney has marketed captain america as Avenger after release of 2012 The Avengers movie. And also as "From studios that bought you Marvel's Avengers!".
While before that it war marketed as "From the Studios that bought you IRONMAN!".
Till the release of Civil War captain america it self became a brand so it was not marketed as above two strategies rather than it was "Divide we fall!", "Captain America 3", while the posters of Civil War almost resembled Avengers type gathering so people were attracted to it. Specially some posters feature Ironman vs Captain America which was also a USB.