What you are doing then is taking a martial drill and turning it into a strength training drill, which is not the same thing even though strength is typically an asset in martial training.
As a martial drill, there is more to three-star blocking than just swinging the arms. There is the engagement of the lower body, using the legs and feet to drive the torso into a rotation, which is what gives power to the swinging arms. But the arms and shoulders themselves should be quite relaxed. Not floppy like Gumby, but relaxed while strong. Using the exercise specifically to build arm and shoulder strength separates the movement from the work that should be done by the legs and torso. It undermines the very purpose of the exercise. On some level it can still be effective, but is unlikely to ever reach its true potential, and you will always be simply working harder to get there and you will miss the process of understanding the efficiency that the underlying method is meant to build, which should be foundational to the entire system, not just the exercise of three-star blocking.
If this all sounds foreign to you, I will say that in my opinion, the instruction found in a lot of schools tends to skip over these details because the teachers do not understand it or at least lack a systematic methodology for teaching it and helping their students develop that understanding. At some point in their lineage and transmission, that particular understanding was lacking and did not get adequately passed to the next generation. Once the knowledge gap has formed, it is very unlikely that people will simply figure it out for themselves without a good teacher who can step in and help them understand it.
Your comment about then doing punches as fast as you can further highlights the issue. I have never seen you train and I don’t know your teacher so cannot directly comment on how you are doing things. But the comment makes me suspect that there are foundational issues you are missing. The point isn’t to throw punches as fast as you can as an exercise. That same work that should be done by the legs and torso that I mentioned above, should be engaged in throwing punches as well. Speed comes later. First, you need to develop the foundation, understand the role that the legs and torso play when you throw a technique, which is something that the three-star drill is meant to emphasize and reinforce. Throw punches with that full-body connection, develop solid technique that is powerful without being tense and without “muscling” it. Later you can develop speed.