If you have already bought version 2, then you can always install it via the App Store. See the following page on how to do it: https://support.apple.com/en-us/HT201272
If you are a new customer, you will not be able to buy version 2. You can only get version 3 from the App Store.
Settings can be transfered from on device to another with the backup/restore functionality. You can also migrate to version 3 by restoring a backup from version 2 in version 3. But you can not install a backup from version 3 in version 2.