If you use the same iTunes account on several devices, you won't need to pay for an additional device the same iTunes account is running on.
In case you have already purchased the full version for an unlimited number of cameras, it is stored at the AppStore.
After deleting and reinstalling the NetcamViewer Mobile software, after an iPhone reset or when installing the software on a new device with the same iTunes account, this setting shall not have been saved on your device.
In that case you can add the third camera after entering the first two and again opt to purchase the full version. The AppStore shall now report that you have already made this purchase and the full version shall be released free of charge. Apple will never charge you any fee twice for the same software on the same iTunes account.