Its not so much to do with the number of filters but the more biological media you can give.
You can have three filters running but only have a small amount of media for each one whereas if you have a filter that houses masses of bio media then thats better, isnt it?
Thats why a filter like a juwel is one of the best going for biological media as internals go. But its downfall is that the pump doesnt circulate the water enough.
Externals are great as well but again you need the circulation or an internal filter that can circulate and use mechanical filtration to catch the larger particles.
Just how i see it though