19.5V to 15V - what's the best way to lower the voltage?
Use a buck regulator is realistically the sane choice. Maybe one of these: -



They appear to cost about £7 from ebay here.

But, as always read the small print and check out a few others. I just googled "15V 3A buck regulator" and this was the first hit and I think it may be a little underpowered so go for something a tad beefier.