If you don't already have a bit torrent client we recommend uTorrent - download uTorrent. After installing a bit torrent client simply download and open the UP_3.0-RC.torrent file (in the above link).
Don't stop your torrent as soon as download is completed ... leave it running for a day or two to help others get their version of UP 3 faster.
Once the torrent download is complete open the folder that the Ultr@Pack 3.0 RC4 install files where store in (in uTorrent this is done by right clicking on the UP30RC entry and select the "Open Containing Folder" menu option). Double click on UP_3.0RC_Client.part01.exe to start the installation process. Use the Change button (in the Choose Destination Location dialog box) to select your IL-2 game folder and continue with the installation.
Download all sixteen files into the same folder. Once the download is complete run UP_3.0RC_Client.part01.exe to start the installation process. Use the Change button (in the Choose Destination Location dialog box) to select your IL-2 game folder and continue with the installation.
I strongly suggest you use the torrent download since it has a far lower possibility of error in the download. It also has a far better stop and resume ability. If you don't already have a torrent client give uTorrent a try (uTorrent Web Site)
If you have a Direct 2 Drive (downloaded) version of IL-2 you will need to install the SAS D2D Patch. Available at SAS D2D Patch
Note: Unlike earlier releases which contained both client and server installers this release is only for the game client. Installation of Ultr@Pack 3.0RC on a server consists of installing the client files (as detailed above) followed by running the server setup/configuration program (UP_3.0_DFServer.exe). The server setup/configuration program is available in the torrent download also.