it depends on how you get to S3. There are programs out there like S3Fox where you have an explorer window where you can do the standard copy, move, delete, etc. You have to enter in your amazon keys before you can access it, so there is no limit to how many computers you can access it from. Just to whoever you give the access keys to.
I use jungledisk for my backups, which encrypts the files before uploading (not sure if S3 fox does that). You can install the program on as many comps as you want, but if you encrypt the files you kinda have to use jungledisk to retreive it. It offers automated backups, versions of modified files, and some other handy features. If you need to access it from multiple comps, unencrypted is probably better.
Check Amazon's pricing page but I think it's $0.15 per GB of storage PLUS some amount per GB of transfer in/out. But they are offering free transfer IN until June. So multiply $0.15 times 3000GB = $450/month.