If you want to store backups and data on the same external drive, you must partition the external drive. To do it, just open Disk Utility (in /Applications/Utilities), choose the external drive in the sidebar and go to Partition tab, so you can create a new partition. Both of them have to be formatted in "Mac OS Extended (Journaled)", because you will be using them for backups (Time Machine or a clone application like Carbon Copy Cloner) and for iPhoto and iMovie libraries.
I'd recommend you to get another external drive, so you have as many space as possible for creating backups, but you should not have any problem with a 3 TB external drive.