Can the High frequency comm system be used instead of adding a 3rd VHF system for ACARS reporting.
Sure, if that's what you want to do. ARINC or whomever your service provider is will need to be compatible as well.
I don't need all the technical info just need to know if it has been done before.
Yes, some AA
international aircraft have used HF
datalinks in the past. I don't think any do today though.
The aircraft has 1 & 2 VHF now but the customer wants to add ACARS and is assuming they will have to add 3rd VHF.
Depending upon certification requirements, you can have the ACARS utilize either of the currently installed VHF radios. Check the owner's certification requirements carefully. It is possible, but may not be legal for owner, user, location, aircraft, etc.