Being in high gears typically means low cadence, high torque, also known as mashing. It works reasonably well on flats, but if you're doing hills it can put a lot of stress on your knees. Starting out, its good to be able to get your cadence up above 60 rpm.

Also, I assume you mean 24 speed (3x8)?
With a triple, the best shifting strategy is to match the front gear to the terrain, and fine tune with the back gear. This allows for quick shifting and you'll hardly be cross-chained (small/small or big/big).

Inner front -- Mostly climbing
Middle -- Small rolling hills, strong headwind
Outer -- Flats & downhill

Infrequent cross-chaining is perfectly fine, the realistic amount of wear is small. So if you're in the largest gear and you come to a small rise, its perfectly acceptable to downshift and go big/big for 20 sec to get up a hill. You don't want to be cruising like that for 15 minutes though.