That's not impatient of you at all - after three weeks you're still right in the phase where your body is getting used to the strain. Your combinations are exhausting because your cardiovascular system and muscles aren't trained for this kind of exertion yet. But that changes pretty quickly if you stick with it consistently.
Realistically, you should expect 8 to 12 weeks before you notice significantly better endurance and the combinations don't wind you as much. A couple weeks after that, strength comes too - meaning you can punch faster and harder. After about 4 to 6 months of regular training, you'll really notice a big difference, not just in your combinations but in your overall fitness level too. Though you'll actually notice some progress much sooner - sometimes after 4 to 5 weeks - suddenly a round that used to completely exhaust you becomes manageable.
The most important thing right now is to keep up the 2 times a week and not change too much. Your body is building its foundation right now, and that just takes time. In a month or two you'll look back and think how easy those training sessions were back then.