The Dropbox restrictions maybe not as bad as they first sound.
The limit of 3 devices running Dropbox applications with a free account applies to the devices that can be connected to a Dropbox product itself, via official Dropbox applications, such as the official Dropbox desktop client or mobile applications.
If your computers are using Dropbox Folder as a syncing method for Moneydance then as you have to have Dropbox Client installed, this count towards the limit. However, if you use Dropbox Connection and don’t have Dropbox Client installed, this does not count towards the limit. This limit does not apply to third party apps that connect to Dropbox via the Dropbox API, such as the Moneydance Mobile App. Your mobile devices would only count towards the limit if you had Dropbox official app installed on your mobile devices for another reason.