Not a direct answer to your question of how to have tripled keys insert certain characters, but have you tried using an input method? For example, C-x RET C-\ german.
An input method is a typical way to use a keyboard to insert characters for a different language.
C-x RET C-\ (translated from C-x <return> C-\) runs the command set-input-method (found in global-map), which is an interactive compiled Lisp function in mule-cmds.el.
It is bound to C-x RET C-\, menu-bar options mule set-input-method.
(set-input-method INPUT-METHOD &optional INTERACTIVE)
Select and activate input method INPUT-METHOD for the current buffer.
This also sets the default input method to the one you specify. If INPUT-METHOD is nil, this function turns off the input method, and also causes you to be prompted for a name of an input method the next time you invoke C-\. When called interactively, the optional arg INTERACTIVE is non-nil, which marks the variable default-input-method as set for Custom buffers.
To deactivate the input method interactively, use C-\. To deactivate it programmatically, use deactivate-input-method.