You will have to purchase a copy of the iLife 09 or 11 disk from a 3rd party retailer like Amazon.com in order to get iWeb 3. It's not available any other way.
Note: if you're interested in iDVD then get the iLife 09 disk. Both disks have the same version of iWeb and iDVD but the 09 disk has all of the previous themes for iDVD.
You don't and you can't. Apple has no longer sells iLife and has not supported iWeb for sometime now.
iWeb has never been available separately so if you want to upgrade to iWeb 09, then you have no alternative but to look on Amazon and purchase either the iLife 11 or iLife 09 boxed set - both of these contain iWeb 09, so you will just have to install this from the disc and forget about the rest.
Either this, or you look at alternative software and re-create your site fromr scratch. Take a look at RapidWeaver, Sandvox, Freeway Express 6, Flux 4 and WebAcappella 4. You can download free trials so that you can try before you buy.