Toyota Repair: terrible gas mileage and very slugish..., piston ring, gas mileage

Question
I have a 1995 sr5 4runner, it has about 230K miles on it, I know a lot of miles, but I had a small tune up done to it: oil change, plugs, plug wires, distibuater and cap, etc, but the truck didn't run much better afterward, mechanic told me that the #3? plug wasn't changed in forever common because were it's at... and that piston ring isn't working anymore probably because of that... The truck is running worse and worse every day with really bad gas mileage maybe 150 miles to the tank, what can I do to help it.  Mechanic said he's going to try to take the cover off and soke the piston ring really well and try to adjust the timing on it, any ideas???

Answer
I think you need to try to find another repair shop that may be more competent, I don't understand the statement about the #3 spark plug no being replaced but anyway I would recommend having a shop check the engine compression first to see if there is a loss of compression in any of the cylinders.