Civil war can't be compared to the Winter Soldier since it'll have Iron man, Black panther and Spiderman in it.
I think Thor 3 compared to Thor 2 will be the real test.
Age of Ultron had Iron Man in it, and you view that as a failure. 95% of people won't have a clue who Black Panther is. If rumors are to be believed, Spiderman is barely in the movie, and obviously not enough people showed up to see his last movie, so he's not a guaranteed draw at this point. Regardless, you can make any excuse you want, but Civil War being massively successful will go against the theory of superhero fatigue, that's all there is to it.
Thor 3 has a disadvantage coming off of what seems to be widely considered Marvels worst MCU entry. So, it doing worse than Thor 2 won't really be shocking. If it turns out good, it'll do just fine, though.
At the moment, though, you're still literally saying
IM3>IM2>IM1
then
T2>T1
then
CA2>CA1
then
A2<A1
OMG! Superhero fatigue! And that's ignoring the fact that they successfully launched Ant-Man, which has done better than Captain America and Thor's debuts. And the massively successful Guardians of the Galaxy.
This is also the case with BvS, how can you compare it with man of steel, when the movie will have over 60% of its runtime filled with arguably the most popular hero on the planet now, Batman, unlike the previous entry.
So, what you're saying is that 2 out of the 3 next big super hero movies are pretty much guaranteed to make more than their predecessors, but those don't count because of reasons. Therefore, the fatigue is real. What will your reasoning be if Thor 3 actually turns out good and does as well or better than Thor 2?