Actually, I'd say the OP's example highlights how a 3rd car almost never makes sense from a purely economic standpoint. He is quoting very bad mpg for the TV, very good mpg for the economy car (is the Cruze really averaging 35 mpg overall, or is that the freeway average?), an incredibly low monthly payment ($120/mo, really?), and even there, only saving $125 a year by his calculations. Note -- I'm not calling bs on the OP's numbers, but they are about as good as you're going to find.
If the OP were to average, say 30 mpg overall in the eco car, 15 mpg in the truck, and/or spend $200/mo on the car payment, he'd be losing money on this setup. I suspect a lot of people do this and think they're saving more money than they really are.
Sure, there are advantages of having the 3rd car, but if economics is the primary concern, I doubt most who go this route are saving much, if anything.
For me, I've used a TV as a DD for years. I only drive 3-4k miles doing non-'truck-stuff.' My commute is only 10 miles each way, and I ride my bicycle weather/schedule permitting. So I guess you could say my bike is my 'commuter car.' It is slow, and wasn't very cheap. But that thing gets great mpg!