It also doesn't need to be an obvious competitor. If you have a strong interest in more niche practices, then the competitors might not be obvious. If other factors have influenced your decision to apply, like secondment opportunities or method in which trainees are supervised, then the firms you are referencing may not be direct competitors.
It is really for you to explain the similarities to why you have chosen the firms you have applied to (which is the purpose of the question). It is sounding out your logic and decision making relevant to your career choices.