The dealership is trying to overcharge you and you were right to question them. The fairing needs to come off for the bar replacement, so charging again to do it for the grips is BS. As you noted, the right side is already wired (the same throttle sensor is used for the bikes with and without heated grips, and the wiring is part of that assembly) so it's just a case of plugging in the wire for the grip. The rest of the wiring is already in the SESG wiring harness, just like the CVO Limited. So all they have to do is fish the left grip wires through the new bars at the same time they are fishing the rest of the wiring through the new bars, then make the connections at the existing connector near the steering head area.
If your dealership won't back off the bogus labor charge, I'd be looking for a different shop to not only handle this particular work, but any future work as well.
Btw, I don't have the labor time standards from Harley for the latest models, but they should be very similar to the 2012 info I do have. The Harley standard to replace the bars on a heated grip equipped CVO Ultra in 2012 was 2.9 hours. That's the total, and it includes removing all the necessary parts and reinstalling everything (including those grips) on the new bars. So if they're charging you 3 hours for the bar replacement that sounds about right, but there shouldn't be another 3 hours for the grips.
Jerry