3 days in Highlands Itinerary

Save the A82 for the return leg of your trip.
First day go via the A9 up to Dalwhinnie to exit towards Fort William.
This way you pass Pitlochry ,there's a distillary at Dalwhinne too.
What time of year are you travelling?

" West is best" or so they say.
For scenery and less traffic a loop out to Mallaig and maybe over to Loch Coruisk blows dull old Loch Ness away ,the A82 between Fort William and Inverness is one of my least favourite places to send time.
Try the Ben Nevis Inn, too I think its Tuesday there's lI've trad music sessions

Day 2 is a heavy day.

Fort William to Culloden is 2 hours minimum, without stopping at Loch Ness.

Culloden to Grantown on Spey 1 hour, longer if you go deeper into Speyside.

Grantown to Pitlochry 90 minutes, and Pitlochry to Fort William via Laggan all but 2 hours.

So with time at each location probably about a 12 hour day.

It is a personal choice if that length of day is viable, as it is not impossible, just far from optimal. Probably removing Pitlochry from the equation and substituting Dalwhinnie Distillery for Speyside (unless your whisky preference is for Speyside!) may be a more manageable itinerary?

With that you could probably get to Glenfinnan for the evening eastbound diesel train at 7pm.

In an ideal world Fort William is not the best base for that itinerary.

Maybe on Day 3 to Pitlochry then through the Trossachs to the bottom of Loch Lomond and across the Erskine Bridge to Glasgow Airport if that is where the car drop is.
For me that is a more scenic route.

Depending on flight time next day, maybe do the Necropolis before the airport?

Day 2 is a heavy day.

Fort William to Culloden is 2 hours minimum, without stopping at Loch
Ness.

Culloden to Grantown on Spey 1 hour, longer if you go deeper into
Speyside.

Grantown to Pitlochry 90 minutes, and Pitlochry to Fort William via
Laggan all but 2 hours.

So with time at each location probably about a 12 hour day.

It is a personal choice if that length of day is viable, as it is not
impossible, just far from optimal. Probably removing Pitlochry from
the equation and substituting Dalwhinnie Distillery for Speyside
(unless your whisky preference is for Speyside!) may be a more
manageable itinerary?

With that you could probably get to Glenfinnan for the evening
eastbound diesel train at 7pm.

In an ideal world Fort William is not the best base for that
itinerary.

Maybe on Day 3 to Pitlochry then through the Trossachs to the bottom
of Loch Lomond and across the Erskine Bridge to Glasgow Airport if
that is where the car drop is. For me that is a more scenic route.

Depending on flight time next day, maybe do the Necropolis before the
airport?

It's a morning flight the next day so the Necropolis is unlikely to be a possibility.

I'm not set on day 2, so I would appreciate some advice on where to go! The commenter above mentioned Mallaig. I'd be open to spending more time in that area, although I think my parents would still want to have something to do in that area aside from just scenery, as nice as that sounds! I was thinking Culloden partly because of the historical aspect etc.

Rather than doing Culloden and Speyside (lots of driving for not much return) how about heading west from Fort William on Day two? The Undiscovered Scotland website has two driving tours suggestions from Fort William and either would make a lovely day out as well as including Glenfinnan viaduct.

The first suggestion is Isle of Skye... It is a lovely drive following the line of the West Highland Railway to Mallaig where you catch a ferry to Armadale on Skey (BOOK thius well in advance!) You then drive across the southern part of Skye - if time allows when you get to Broadford, it is well worth making a detour NW along the A87 following the coast to Sligachan for views of the Cuillins (32 miles round trip) before heading back to cross the bridge back to Kyle of Lochalsh on the mainland. You pass the iconic Eilean Donan castle on the way back (the one that appears on all the tins of shortbread - the outside is fantastic but don't bother with the inside which is disappointing) and also the Commando Monument at Spean Bridge.

The second suggestion is Ardnamurchan peninsula which is all about scenery through one of the most isolated parts of Scotland and also brings you back past Glenfinnan

Rather than doing Culloden and Speyside (lots of driving for not much return) how about heading west from Fort William on Day two? The Undiscovered Scotland website has two driving tours suggestions from Fort William and either would make a lovely day out as well as including Glenfinnan viaduct.

The first suggestion is Isle of Skye... It is a lovely drive following the line of the West Highland Railway to Mallaig where you catch a ferry to Armadale on Skey (BOOK thius well in advance!) You then drive across the southern part of Skye - if time allows when you get to Broadford, it is well worth making a detour NW along the A87 following the coast to Sligachan for views of the Cuillins (32 miles round trip) before heading back to cross the bridge back to Kyle of Lochalsh on the mainland. You pass the iconic Eilean Donan castle on the way back (the one that appears on all the tins of shortbread - the outside is fantastic but don't bother with the inside which is disappointing) and also the Commando Monument at Spean Bridge.

The second suggestion is Ardnamurchan peninsula which is all about scenery through one of the most isolated parts of Scotland and also brings you back past Glenfinnan

Thanks so much for this suggestion and even including a link! I will definitely consider this. I did want to go to Skye but was worried maybe it was too far.