act 3 is the final act in the horde campaign, where Rexxar, Rokhan, Cairne (and chen stormstout if you have him) attack theramore. the second part of the mission is a invasion on 3 fronts of theramore isle, with an ending fight against the leader of theramore, Admiral Daelin Proudmoore. i think it's a pretty nifty battle.
my brother has the game, but it lies at his GF currently. i asked him if he could bring it along with him, but he seems to have forgotten it. and i still need to install it anyway. i just want to see the mission again, since i can't seem to make DotA for SC2 work.
Roundabout way - Does your brother have his war3 cd key linked up to battlenet?
If so, if he is comfortable with sharing his account info with you (possibly change his password beforehand?), you can download warcraft 3 from the battle.net website and install it. I think the torrent files are linked to the computer that initially downloads it (and checked frequently) so he couldn't just send that to you.
If I want to see a game I really don't want to buy, I will watch it on a Youtube Walkthrough in a dual screen. There is usually a Walkthrough for most popular games, so just search "Warcraft 3 Walkthrough" and skip to like video 30 or so and there you go.