From my experience, the reason why so many nail techs find 3D acrylic sculpting so difficult is often they are using a brush that simply isn't good enough and doesn't provide sufficient material control. Paying a lot of money for a brush from a well known company does not necessarily mean it is good!
The correct brush size to use is a #4. A good brush should have only the best quality Kolinsky hair (there are many different qualities) and it must have both the right length and enough stiffness to allow you to sculpt the material in fine detail.
The #4 brush shown above is one of our most popular brushes. It was also one of the most difficult to develop, but the end result was worth it because experienced nail artists just love it!
So if you are learning how to do 3D sculpting and not getting the results you want, take a closer look at your brush. You can ask for recommendations from experienced nail artists on forums such as Beauty Tech or Nail Geek.