About the H-3... The problem is that you need a crazy accuracy. And even with this you still are human, you'll still have to react at surprising moves and unforeseen things happening all the time. Once you screwed up your good chance the weapon becomes unreliable and the opponent almost always have the advantage...
And let's say you do have the accuracy, you're probably better off with a squiffer or even other chargers weapon.
If you want power you'll still be better off with pros or .96, as you can chain multi kills way easier. In a competitive game, it will be difficult to chain kills without a special with a H-3, as you suffer from a lack of flexibility (like a non-shooter weapon [blaster, buckets,...], might be even worse than a charger [surprisingly chargers can be quite good at moving around]). Then if you're not a power, are you defensive? But you'll still prefer a .96 or a longer range. Or are you a support? You'll prefer other weapons... again.
So, the H-3 is harder to use than the others, and doesn't reward you for trying... Plus it's kinda annoying to play... It's no surprise you don't see it that much. L-3 doesn't suffer that much as its range is kinda interesting and its flexibility problem is less important.
Damn, I kinda want to play it now.