i saw the first Guardians and the Doctor strange films( both of which I liked) as well as the first Iron Man film. Superhero teamups have had a mixed success rate historically. Take the 90s batman films post Returns. Bat nipples anyone? Bat credit card? Bane being a mute henchman, when in the comics and Animated Series, he is one of the Dark Knight's greatest foes? You get the idea. I think Marvels just done team ups better than DC, granted most of my superhero stuff comes from DC.
Oh, boy. Okay, then... Heh heh... Well, you've certainly seen three of the "Must See" ones. Alright, before I answer anything else, here's a list of the films you'll need to see before being ready to see "Infinity War."
Ironman 1
Thor
Captain America: The First Avenger
The Avengers
Ironman 3
Captain America: The Winter Solder
Guardians of the Galaxy
Avengers: Age of Ultron
Captain America: Civil War
Doctor Strange
Guardians of the Galaxy, Vol. 2
Spider-Man: Homecoming
Thor: Ragnarok
Black Panther
(In that order.)
If you skip these, you're not going to get a lot of what's going on in "Infinity War." Like I said, it's like the seaso finale of a TV show. A TV show like "Walking Dea" or "Game of Thrones" in the sense where the individual episodes leading up to it matter. Not the tone, though. Good lord, not the tone.
Superhero team-ups: Well, so far, the Marvel Cinematic Universe has made their team-ups work consistantly. Even the least liked of the Avengers films (Age of Ultron) has generally favorable reviews.