Argyll and Scottish Highlands

With only 3 days you are not going to be able to cover both the northern part of the Highlands and Argyll.
Oban would enable you to see the Glencoe area, Loch Lomond and perhaps do a day trip to Mull.
Inverness (or Nairn which is prettier) would open up the northern Highlands and Speyside.
Fort William for Glencoe, the Nevis mountain range, the road to the Isles (Glenfinnan), Loch Ness.

You've left it extremely late to book accommodation for this summer. Bear in mind too that August is peak midge season, so bring repellent.

I'd add follow Skyegirl's advice, and say almost look in those three areas and then book as soon as you can and take it from there. Remember Highland Council on its own is roughly the size of Belgium, and whilst Belgium has a comprehensive road network Highland has a road network, and both council areas have limited high speed roads,

If you are based in one of the three areas mentioned, there is more than enough to do for the days you have.

Have a think about Isle of Arran which is often described as Scotland in miniature. Adrossan is just over two hours drive from Peebles. and the ferry takes about an hour. Timetable here.

There are lots of opportinities for horseriding, kayaking and walking.

Ideas for food here.

Check out ideas of things to do and see by following the links on the map of Arran here.

You've only three days so don't waste time driving long distances. Arran ticks all your boxes

Thank you all for the great suggestions and advice.
I have (cancellable) reservations at Loch Lomond (2 nights) then on to Cairngorm area (1 night), finishing with Edinburgh for several nights. Don't know if that's too ambitious of an itinerary? or if I should do 3 nights in one place and satellite out for day trips, before finishing in Edinburgh?

Personally I would not stay around Loch Lomond for two nights. You are quite far south in terms of the Highlands. That's why I originally suggested alternatives (see above).

Thank you Skyegirl, most appreciated!