Back up from Time capsule to External Drive

I want to move all of my media from my hard drive on one macbook to my time capsule. i currently use the time capsule with time machine to back up my hard drive. but, i have multiple computers. at home, i have a macbook and a macbook air. at work, i have an imac. i'd like to keep my photos, videos and music on the time capsule's hard drive, so i can access it from each computer. i have the time capsule registered with mobileme, so i can see the contents of it from each computer. presumably, then, if i put the media on that disk, i can access it from each location.

but, now i want to have a back up of all of that media. i have an external hard drive i can plug into the time capsule. my question is this: is there a way to use the time capsule as i have described and then have the time capsule's contents backed up to the external hard drive? if not, is there a way to put everything on the external hard drive, connected to the time capsule, access the content from all three computers, and then have the external harddrive back up to the time capsule?

any help would be really appreciated!