WhatsApp requires iOS 4.3 at the bare minimum, meaning OP cannot simply go to the AppStore and download it, free or not.
You'd have to upgrade your iOS to a later version that supports it. If you don't have blobs saved for your specific device for a specific iOS, your only option of restoring on the 3GS would be 4.1 or 6.1.3, neither of which requires blobs. But since WhatsApp is 4.3+ only, you'd have to restore to 6.1.3 if you don't have blobs