There is no trigger for that. Best you could do is act on the after user update trigger and check if the field group fields value changed by comparing it to its old value. Example of this as follows.

Global
Triggers: onAfterUserUpdate
User: Automatic
Access: Everybody
Conditions (custom value)
1: [var1_FIELD_NAME] Not Equal To [var3_FIELD_NAME]

Replace FIELD_NAME with the actual name of your field. This will result in your auto action executing if that fields value has changed. Use the following list of triggers if you need this for frontend and backend registration and profile edit.

onAfterUserRegistration, onAfterNewUser, onAfterUpdateUser, onAfterUserUpdate