-How many Invoices do you think are missing?
-What do the missing Invoices have in common? Are they sequentially numbered or the same date?
-Does it look like the missing Invoices could represent a batch?
-If so, has an A/R batch been deleted? Print the Batch Status in A/R and look for a Deleted (or Open) batch.
-Did you experience any errors during Day End processing at any point?
-Have you restored any data files from backup recently? If so, did you restore all files or only A/R? A/R, I/C and O/E should be restored together.

It doesn't sound like a backup/restore problem. Did other invoices for the 15th and 17th get posted OK? If so, it doesn't sound like a deleted batch.

Other than being to the same customer, do those 3 invoices have anything else in common? Is there anything unusual with the dates?

Have you run a Data Integrity Check on A/R? Choose "File" then "Check Data Integrity".