Iron Man 3 can be streamed via Disney Plus along with all the other Marvel movies, but you'll also find the movie streaming via STARZ. We'd recommend trying STARZ for a few months as there are some good movies on there, you can do so via Prime Video.
Currently, you can stream Iron Man 3 with Disney Plus. You can also rent or buy Iron Man 3 to watch online with iTunes, Prime Video, Microsoft Store & Google Play.
STARZ is a streaming service that has quite an impressive collection of movies ready to watch, including Iron Man 3. Another great thing about STARZ is that it can be tried for free via Prime Video for 7 days, so if you want to watch movies for free via STARZ you can simply use the trial. Use the STARZ trial to stream Iron Man 3 via Prime Video here.
Disney Plus is another great platform to stream Iron Man 3 via because you can also stream the 1st and 2nd Iron Man movies via this platform.