H3 was my favorite in terms of sandbox where just about every weapon had an advantage that the others didn't. On the whole, each weapon was effective in a variety of situations, but for the most part each weapon had a single role that they truly excelled at.
I don't know what game you played, but in the Halo 3 I know, a large portion of the weapon sandbox was practically useless; that is, most weapons that were not power weapons or the BR. Dual wieldable weapons were weak because Bungie was afraid that dual wielding would be too powerful. Of course there were some weapons with redeeming qualities, such as the Mauler which you could use to take shields down if you got close enough, and the Plasma Pistol which even in its probably worst iteration at least had the trusty overcharge. But weapons like the Magnum, Spiker, SMG, etc. were something you might as well just run past. Any of these could be beaten by a BR in any situation.
Halo 3 didn't have good sandbox design, because Bungie was afraid to give any weapon appropriate power. Even the BR had spread for some incomprehensible reason. It was also kind of lazy in that weapons didn't really have that many unique properties. Now, this also holds for Halo 2, but for example the stun effect of plasma weapons that we had seen in CE was completely absent.
I'm not trying to say that Bungie did everything wrong with the Halo 3 sandbox. In terms of power weapons, they actually managed to introduce two weapons that have since then become pretty iconic: Spartan Laser and Gravity Hammer. It's just that when it comes to the niche weapons, the Halo 3 sandbox was a nerf job of the Halo 2 sandbox, which in turn was a nerf job of the Halo CE sandbox.