You can always add an external USB or eSata enclosure if you desire or build your own NAS box. You can also add more sata ports with pci, pci-e cards as well.
You certainly could find a more efficient solution then backing three drives up to three external drives. A raid solution would be more efficient. There are several software varieties, each with pros and cons. FlexRaid, unRaid, SnapRaid, Windows based raid, etc...
Generally speaking, I recommended a dedicated box for media storage and a separate box for HTPC, gaming, general computing, etc... build a nas can be quite cheap (minus the cost of drives) if you're willing to go that route.
A specific build would need some more specifics from you, including what assets you have, what you're willing to spend, and if you're willing to learn an OS or software function. Like everything in life, there are choices. Buying a NAS like the synology or drobo are certainly easy, but you will overpay by significant amounts for what even a novice computer user can manage and understand with a little bit of effort.