Hey!
I would say that this isn't actually too different from a standard "3 skills of a good trainee" question, and the core foundations are generally the same.
The difference is likely in the framing - for a "commercial trainee", you would place more emphasis on commercial awareness, client focus and practical judgment / advice. It is about showing that you don't just complete tasks accurately, but that you understand the client's business objectives and think about the wider commercial impact of the work that you do.
Therefore, you don't need completely different skills - I would just write your answer with a slightly more commercially focused angle on the same core competencies