Meh, if your 3 year old will sit through it, go for it. The Avengers films are really little more violent than the Star Wars films (rated U back in the day), Watership Down (also 'U') or the Indiana Jones films ('PG' but marketed to children).
I would love to take my youngest to the Marvel films but I know that she would get 45 mins in and get restless. I took my 5 year old nephew to Black Panther (loved it) and my (then) 4 year old daughter to Guardians of the Galaxy (loved) and Jurassic World (she loved it, I hated it).
The most distressing experience either of my children have had at the cinema was when our local had a showing of The Lion King (rated 'U') - my oldest was extremely upset when Mufasa died and still hasn't really got over it. Ratings aren't everything.
Feel pretty sorry for the 10 year olds whose parents on this thread won't let them watch these - sounds grim.