From the BBFC Insight: AVENGERS: INFINITY WAR is a US fantasy film in which an army of superheros must work together to stop an evil foe from destroying the universe.
Violence
There are frequent scenes of moderate fantasy violence in which various characters with special powers fight each other or battle an army of aliens. The violence includes use of shooting with futuristic weapons, punches, kicks and blows from other weaponry.
Threat
There is a scene in which a character is suspended by wires and pulled apart by a villain trying to extract information from her, but the torture is fantastical.
There is a middle finger gesture and incomplete uses of 'mother' and "chill the f out". There are also milder terms such as 'God', 'douche', 'shit', 'crap', 'butt', 'bastards', 'jerk', 'hell', 'ass', 'damn', 'piss' and 'screw'.
I wouldn't want a 3 year old listening to this kind of language, regardless of whether or not they have watched previous films. Your child is still at nursery, presumably with another year to go until they are old enough to go to school. You have obviously already made up your mind that your 3 year old is far older than their years, but I think it's actually really sad that you think it's remotely appropriate for them.