Captain America: The Winter Soldier was (apparently) a GREAT movie ('cause I hated it). The only thing that was heavily noted was the feeling of the movie. It did not feel at all like a sequel to the great 2011 reboot, but instead felt like a simple Marvel movie that used some elements of the past film(s), it felt like it should've be called "S.H.I.E.L.D: Who Do You Trust Pt. 1 - The Winter Soldier" (hey, that's a good title!). However, The Avengers: Age Of Ultron is each time closer to us and it has already been confirmed that it will be directly followed by the Third Captain America movie. It has also been confirmed that the Civil War will be adapated and that Robert Downey Jr. will return as the man in the Iron Armor (Iron-Man (if you didn't understand)). This means that Captain America 3 will be a sequel but a...
Spin-Off From The Avengers Series:
The Avengers is one of the 4 series in the MCU and with Captain America 3 having the Civil War storyline, Iron-Man, this means that because it will directly be the continuation of Age Of Ultron, it will probably feel more like a spin-off. Yes, it is the third Cap movie but Iron Man 3, Captain america: The Winter Soldier and Thor: The Dark World don't seem to affect the continuation from The Avengers (2012). What I'm trying to say is that Captain america 3 will feel much like a like a Post-Age Of Ultron or a Pre-Infinity Gauntlet.
Alternate Titles (So You Could Comprehend)
Captain America 3 will be kind of like the actual 3rd Avengers series movie so to let you understand more, these are some titles (that will not be used) that might help you comprehend this idea.
-Marvel's Civil War / Captain America Vs. Iron Man / The Avengers: Age Of Ultron Pt 2.
Captain America 3 can be considered half a sequel and half a spin-off. Or at least I will. The Avengers: Age Of Ultron will arrive May, 2015 and it's sequel/spin-off/next installment will be released in 2016.