This late in the game for FC3 and after so many months since the original ISOs were published, you will have to download about 1 GB of updates given that you selected 'everything'.
Given how easy it is to install things in Fedora (Yes, I said it) using yum and the 'add/remove' tool, It is better to install just what you need when you need it instead of having the kitchen sink that will just bug down your system both on the resources it uses and the time it takes to update.
Having said that, when I started with FC3 I decided to do an 'everything' just to see what it had. after a couple of hours of playing around, I started from scratch with a more limited install.
Regarding the question: Which updates should I select? My recommendation: Select all the updates, It only shows updates for things that you have installed.
Regarding the question: How to download only once for all my computers?
My recommendation: Create a local yum repository and update with yum
Here are the instructions
and
Here is a quick HowTo for yum