I need to backup all my computer data. I'm thinking of buying 3 External Hard Drives and connect them to Airport Extreme. Can I do this?
The constraint is be the number of ports available on your USB hub.
Any connected hard drives should appear in Mac OS X. Use a self-powered usb hub and connect hard drives your hub. Only then will multiple external drives have enough power to run.
Also see store.apple.com for related questions.
As shown on the Apple website, the Airport Extreme has only one USB port:
However, you can use a USB hub to connect multiple devices to it.
(I also wouldn't be surprised if Apple eventually creates a Thunderbolt-compatible version of the Airport Extreme — and Thunderbolt devices can be daisy-chained, so you might be able to connect more than one hard drive at a time.)
For a single hard drive, you might also like the Time Capsule which has an integrated hard drive.
For a more flexible and heavy-duty solution for multiple drives, you could use something like Drobo.
Yes you can - Apple's web site says "multiple printers, multiple hard drives" and mentions using them with a USB hub.
Don't expect fancy NAS or RAID features though, you will just be able to share the existing partitions.
The problem is how much power USB on the Airport Extreme supplies.
If you had one external drive, then 2.5 inch drives will be fine with the power they get, 3.5 inch drives usually are not. I had one drive that came with a special USB cable with two connectors, one for data + power, the other to get a bit more power. That won't work obviously.
For multiple drives, you need a hub. For multiple drives without their own power supply, you will need a POWERED USB hub, that is a USB hub with its own power plug. Be careful when you order one, because some people sell their unpowered hubs as "USB POWERED", which means it tries to get its power from the Airport Extreme USB port. That's not going to work.
If you buy 3.5 inch drives that usually come with their own power supply, they might be fine with an unpowered hub, but I wouldn't take the risk and get a powered hub.