There is significant overlap and you shouldn't cross chain, so practically speaking, your typical 3 x 8 speed has anywhere from 14 to 16 useful gears. The triple offers a wide range of gears, some of which you might not use on every ride, but might just save your legs and behind on hills, or in the wind. I use a 3 x 9 speed and while I used 5 or 6 gears on the middle chainring most of the time, there are hills where I use 3 or 4 gears on the small chainring. On hills, I will often start on a gear on the small chainring that overlaps with a gear on the middle chainring. That way, if the hill gets steeper, I have 3 or 4 gears left on my cassette. Don't use the big chainring all that much, but it is useful on long flats with a tailwind or on downhills, particularly on group rides where I find myself close to spinning out on the middle chainring.