What is this trio of fins at the back of a T-38?
There are a couple of possibilities, both out of the T-38 Flight Operations Manual.

It's a nozzle for a smoke generator.  This was an option on the T-38 airplane to generate a smoke trail behind the aircraft for training purposes.

If possible, inquire if that aircraft was an AT-38B.  Those aircraft apparently had an optional 'ballast box' in that region, which I'm assuming was some sort of a counterweight placed there in conjunction with the bomb racks for practice bombs and other range equipment.

As to the vortex generators?  Who knows!