How to install Dropbox on iPhone 3GS? How can I install the Dropbox app on an iPhone 3GS running iOS 6.1.6? THe newest version of the app is iOS7-only.

I've heard that if the app has been installed under your account, the App Store will let you use the last compatible version, but I have no devices running iOS7 to add it to my account with.

How can I get the Dropbox app onto this phone? The iPhone is jailbroken.
Indeed, it will let you download the last compatible version if you already have the app in your purchase history. You can add the app to your purchase history using iTunes on your computer, since iTunes is version-agnostic. Once you've downloaded the app on iTunes, it will be in your purchase history and attempting to download it again on your device should prompt you to accept an older version of the app.