What would cause a drop in gas mileage on a 2001 Avalon, about 3 mpg,

Spark plugs won't really lose you 3 miles per gallon. It's not a bad idea to replace them at this point anyway as a proactive measure. Ensure your driving conditions have not changed. Cars are pretty static in performance, but obviously economy varies depending on environment and driving needs. Temperature outside can figure in, also the blend of fuel used (there are 6 blends of fuel year round, the current blend could be more volatile than the weather allows for, causing less economy). If there was something actionable, then you would normally have a check engine light illuminate and your technician would find some kind of correlating problem.