Hi,
Scotland is a small country, and you can see quite a bit in 3 days. If it were me, I would head to the west coast.
I'd recommend you head from Inverness to Ullapool, which is only 1 hour by car, and then go up to the Inverpolly region. The mountains are spectacular there. Stac Polhaid, Cul Mor, Suilven etc. There is a nice road that takes you round the back of the mountains. Well worth a visit.
From there, just start to head south on the west coast and you'll be immersed in some beautiful lochs and mountains. Ending up in Glen Coe.
Alternatively, if you want to concentrate your time in a location, then I'd just head from Inverness down Loch Ness towards FortWilliam, then into Glen Coe. It's a fabulous region and comprises of some of the most atmospheric landscapes in scotland. Admittedly, they have been photographed to death, but well worth a visit. The journey in a car would only take around 4 hours. Lots of camping / youth hostel and B&B options.
Have a great time. The only thing that will be against you is the weather. Take plenty of waterproofs!