98 V6 motor help!!!!

Whats wrong with it?

You're doing the most important thing- changing the oil regularly. Neglected oil changes are probably the number one killer of engines. Many people (myself included) go with a 5000 mile interval, but 3000 definitely won't hurt anything. I would suggest using off the shelf oil and filters- that means doing it yourself or taking it to the dealer. The stuff they have at oil change places is really sub-par in some cases.

I would stay away from additives (with the possible exception of a Sea Foam treatment and an occasional bottle of fuel system cleaner). I would definitely not use those seal "conditioners". They cause more seal problems than they prevent. If the engine doesn't have any big oil leaks, why mess with it?

Other things you can do are a brake fluid change (hasn't ever been done on most cars) and coolant change. If it's an automatic, a tranny flush and fill would also be good.

You're on the right track to keep it on the road for another couple of years! Most cars are junked not because they're just worn out, but because neglect of routine maintenance has caused early systems failures.

Do your exhaust then your suspension bits first. Not really a believer in cracking open the engine unless something has gone.

If your still interested, I did a tune up at 200k miles and the following changed the overall performance incredibly:
Replace the fuel filter. I had no idea it was robbing performance so much.
Do the Seafoam treatment. Soak the upper intake plenum, use it through the pcv intake, and put it in the gas tank. Those two easy and cheap preventive maintenance procedures will make you happy when done.