You can use instance variable also, its does not matter really.
If you hold a button for the attack to happen, for example lets say key f on the keyboard and it takes 3 seconds to charge. You will have to add keyboard object in your project, after that in the event system add a condition on key f hold, as action add wait 1 sec, add 1 to instance variable (which is 0 on start, lets call it combo).
Now for the attack it self, add a condition combo => 3, ass action add your animation and the attack you want to happen, and reset the combo to 0.
You will also need a Boolean condition on the charging event, so when the combo attack is getting executed the charging with stop.