Because it's a hassle having 3 filters that would use up more power than going it safe and buying 1 really good filter that is only a little bit more expensive and will probably perform better for longer. See what I mean?
1. These filter uses very little power - unless you're also on planning to use UV constantly
2. Maintenances is fairly easy and only need to be performed less since there are 3 of them spreading out the mechanical filtration workload.
3. Yes, buying a good name brand filter like a Fluval or Eheim may be worth it in the long run, but I don't think that Aqua One qualifies to be in the ranks as Fluval or Eheim.
4. Just trying to help you out and save you some $.
5. Good luck