You can't play dota without installing warcraft 3.
Unfortunately no.
Try rapidshare
Start>Programs>Warcraft 3 if it's not listed there re-download warcraft 3
Other than possible torrents, which are very likely to give you viruses, there are no places where you can download halo 3. If you are feeling lucky, try torrents. Otherwise, it is smarter to buy it yourself.
Yes
you can download it from the torrents through torrentz.com.make sure there r more seeds than peers for faster download
No it does not.
If you already have a valid serial number you can add it to a battle.net account and download it there :)
You can't watch dota trailer without installing warcraft 3. But, you can watch it in the internet, specifically on youtube.
You can download the Games Through torrents. Torrents for games are usually illegal and hence i suggest you buying the game. Also There are new GTA games with much better graphics.
Blizzard offers a free download for it, if you have the CD-Key
It's a mod you need to download it from the hivewrkshop
Log-in to your account management on the warcraft site and scroll to the bottom where it says download client, their is then a link for PC & MAC :)