Open up the owners manual, most of the required service intervals will be listed there. Though 3k mile oil changes are excessive by modern standards, but if you have an older car they allow you an opportunity to assess its condition on a regular basis. I would never change oil that frequently unless the application required it or there was an existing condition that necessitated it, but that is just my opinion. Your certainly not doing any harm...assuming the maintenance is done correctly.
I'll recommend the following website if you want to understand more about vehicle lubrication. Motor oils have come a long way, and are more robust and longer lasting then ever before. Used Oil Analysis - Bob is the Oil Guy - Bob is the Oil Guy
As for spark plugs, it depends on the type of ignition system and the type of plug. Your 00 GT specified plug changes at 100k miles. Modern coil on plug ignitions using iridium or platinum tipped plugs can go upwards of 100k on the factory set of spark plugs. However, inspecting them at intervals of say 20-30k miles, will allow you to keep track of their condition AND it usually prevents them from getting seized in the cylinder head like is sometimes the case at 100k mile "tune-ups".