Freeform 4 (free version) supports plugins in the “Email Message” notification field.
It should be possible to use one of Low Variables tags instead of the simple global variable syntax:
{exp:low_variables:parse var="my_low_variables_var"}
Or you could write your own simple plugin that outputs whatever you want.
But I dont’t know if plugin usage in notifications works with Freeform 3. It’s worth a try I would say.
Another way could be to use hidden input fields that contain content for the notification mails. You can use Channel data, Low Variables, …:
<input type="hidden" value="{my_low_variables_var}">
This isn’t very secure in Freeform 3 (or earlier versions of Freeform 4) because it would be easy to manipulate the value of those fields prior submit.
But Jim Wyse from Solspace postet on December 7:
Freeform 4+ now stores the information that was in hidden fields prior
in a parameters table and calls them with the parameter ID, so those
are no longer exposed as hidden fields.
This is cool.
If there aren’t any reasons to not upgrade to Freeform 4 (free version), I would suggest the upgrade and one of the two solutions above.