then why do most typical cars have 2 cats??? the 3rd cat is a waste as mentioned by master with simple up keep. no to mention you could get cats that do a better job then the stock ones and have 2 of them so it would all be equal.
Master486 is correct about the 3 cats all being 3 way cats, but the 3rd cat was put on by ford to further reduce tail pipe emissions for this vehicle to make it exceed
the 50 state emissions qualifications, your state (cali) is especially strict about their emissions standards.
If I lived in cali I would bolt on an aftermarket cat back system and be done with it because of all of the emissions bull crap out there.
Yes there are cats out there that can do a better job than the stock ones, yes there are cars out there with 2 cats from the factory, but most 4.6 liter fords came with 4 cats (mustangs, town cars, crown vics, etc.), the reason the thunderbird has 3 cats instead of 4 was more than likely to make the routing around the gas tank that much cheaper and easier.
The third cat is a three way oxidizing cat as I mentioned before and the front two are the primary cats (also three way) which legally in all states but cali may be replaced with 2 way cats if replacement is required (federal minimum mileage for replacement is 50k for 95 and older, 80k for 96 and higher), the sole purpose of the additional cat was to further the reduction of HC's and CO's from the tail pipe emissions.
Do what you want with your car. I feel that the magnaflow 3 piece set up is a great solution to the expensive ford replacement alternative and for most it flows above and beyond their needs.
Modify your car in whatever way you like but be prepared for any and all legal ramifications.
-1996 Pearl White Thunderbird 4.6 Sport 4v with many mods:
Current Best: 13.583 @ 103.74 10/2014.
227hp/241tq measured on calibrated mustang dyno, tested at full vehicle simulation with inertia enabled, no number skewing.
-2001 White Ford F-250 Super Duty 6.8 V10, 4-door, 4x4, Limo Tint, BFG KO2's, Bilstein 7100 Remote Reservoir Race Bypass Shocks, 6" Spring lift, Kenwood Double Din, K&N Intake, FlowMaster Exhaust
Amber_Murder's Daily Driver.
-2005 White GMC Sierra 3500 6.6 Duramax, Dually, 4-door, 4x4, Limo Tint, BFG Commercial T/A's, Rancho RS9000XL Shocks, Kenwood Double Din, K&N Intake, Banks Monster Exhaust
Chris_Murder's Daily Driver.