This week Marvel announced five new release dates (but not films) through 2019. Then this morning they did something unexpected: they added another film to the calendar. The new movie will be out May 4th, 2018 - which means it's probably Avengers 3 and the official end of Phase Three.
That gives them three movies in 2018, following up on three movies in 2017. It looks like Marvel is doing three movies a year now.
And this also opens up lots of new opportunities for new franchises. In fact, if my math is correct, this could mean two new franchises by the end of 2018: Captain Marvel and Black Panther? We should know more come Saturday, when Marvel does their big Hall H presentation.
The timing of this is interesting: it looks like Marvel was waiting for Sony to announce a date for The Amazing Spider-Man 3. They announced 2018 this morning... but didn't stake out a specific date. Marvel apparently jumped as soon as they saw Sony wasn't being specific.