I'm a technician myself, owned my own shop for over 10 years, and if you had 3 mechanics look over and couldn't tell u definitively that your cats out, in my opinion they aren't very good. It takes a simple laser temp gun and a helper to check Cats to know for sure if they are bad. But for you I hope 1 of the 3 are right, cause otherwise you just wasted money, I'm just not one for doing that. Diagnose, and do it right the first time, my motto.I had diagnostics done on every system that affects mpg. O2 sensors, Egr system, batteries, plugs, wiring, valves, and all are perfect. Three mechanics have told me that only leaves cats, which have been throwing their dreaded code more and more frequently. I've ordered new ones and all 3 sensors and if that doesn't fix it, you guys will be the first to hear about it!
Anyway, wish you luck and post an update asap!