Trading three tanks for a one 6ft tank

[Galantspeedz]

I think he meant you can have more varieties of fish with 3 different tanks as compared to 1 tank...

While a bigger tank means you have more varieties to choose from but is limited by compatibility

 

[Backfromthedead]

I'm afraid I'd have to disagree with your statement of being able to keep a smaller variety of fish with a bigger tank. You would actually have a much greater selection of fish to choose from and in fact you could now have fish that would not be able to live with other fish together. What are the chances of a flowerhorn and a red devil living together peacefully in a 75 gallon and a 180 gallon?

Mine are doing pretty well in a 150, but i think its dependent on your particular fish really.