Hi Philip,
Here is a possible solution:
1. Create a scheduled event / After commit event
2. In the SCE/ACO: try retrieving all 3 files from the database with a constraint on size > 0 and HasContents = true and ofcourse a relation with the customer. The size of each file will be zero if the file isn't uploaded (commited to the database) yet .
3. If the list has a count that is equal to 3 --> send the email.