Do you ONLY want one email when all three files have arrived?
or asked another way (to stress another part of the question)...Do you ONLY want one email when all THREE files have arrived?
Scenario 1
If you want the software to send one email/file if the files are spaced out by X sec but send one email when they are not, then you can accomplish that with a trigger deactivation.
Trigger on, for example, *.txt and Under the "Expires" tab you set the trigger to expire after one time
Then have a Task (probably at the end of your job) to re-enable the Trigger but with a "Wait" as the task right before that
It would run like this
Files Dropped
Trigger Fires once and becomes inactive
Task 1 - Email
Task 2 = Copy task
Task 3 = Email
Task 4= Wait for 60 seconds
Task 5 = Copy Task (again, because more files may have shown up in the 60 seconds)
Task 6 = Reactivate the trigger
Now when multiple files are dropped in any one minute period only one email and one set of tasks are run. If more files come in within that 60 seconds they still get copied
Scenario #2
You want to ONLY copy and email and whatever else WHEN all 3 files exist
That is much easier. You set the dependency on 3 files to exist. The job will not run UNTIL all three show up
Go into your trigger, set the THREE triggers for the three specific files and then click on "Dependencies"