In order to choose a custom notification sound in Mailplane 3, you need to add it to your system sounds first.
Open Finder, go to Go > Go to Folder (or press
Cmd-Shift-G) and enter the following path:
Copy and paste your custom sound file into to that folder.
Restart Mailplane 3, go to
Preferences > Accounts > Notification and choose your newly added custom sound in the
Play Sound list.