3mm is a large vertical impaction. I feel for your situation I have something of a similar situation, mine was more like a maximum of 5mm impaction but all posteriorly. It's a horrible situation to be in. I went in like you poorly informed, so
I'd say jaw surgery is still worthwhile for many but all practical steps need to be taken to ensure a good outcome and protection in the case of a bad outcome.
Do they acknowledge the 3mm impaction? In my case they will not, this is generally because they are only concerned with the anterior height of the maxilla ie incisor show. All of my impaction was posterior so I have taken on the appearance of a vertical deficiency and bite is very uncomfortable.
What are your plans for the future?
Regards
Ben