With most rods, the test curve is an indication of the best casting weight. 2.75lb TC rods are best suited to 2.75-3oz leads as you have discovered.
Whether you can cast 120yards is down to a number of things:
Your reel. The reel needs to be big enough to hold the line. A 10000 size Shimano baitrunner can cast 120yards, but not with 15lb/0.35mm line.
The line you are using. If you are using 15lb line then the reel needs to be Big Pit style. If you are using a 10000 size baitrunner, then with 8-10lb line (and shockleader), you could cast 120yards with 3oz lead.
Your casting style. 120yards is a BIG cast, not a standard distance, it is a massive cast that takes practice and having the correct style, and possibly even the right weather conditions.
Most anglers who think that they are casting 100metres are probably closer to 80m. They very much overestimate the distance. It is only unless you can measure that you can be sure.
Finally don't aim just to be a long caster, far more big fish are caught in your near side margins than at long distance, and your margins are often an ignored area