I have a user who is in both Group1 and Group3. Will that user only be able to data with value EUROPE for %ACCESSKEY or will they be able to see data for EUROPE and US?
If you really want to know you should simply test it.
In general every record in the section access table that grants access is additive. A record cannot remove or replace access that is granted by another record.
Since you use * for %ACCESSKEY in the Group3 record that record will grant access to US as well since that value is listed in a record elsewhere in the section access table.