Most backup solutions only attach to one server and you send files from other servers to that one, or there is a backup agent that moves files.

Arcserve, Veritsa, BMC, all large backup vendors make software for this. Unless you can afford a high bandwidth offsite link, go with tape.

I'd set up a process to copy data from the SQL box (full, log backups) to one of the other boxes. Perhaps the same thing for the web box and then have tape software move those files to tape.