When I had my Cavalier, I went 5K between oil changes, and, now, the service department dealership I work at sets customer oil changes at every 6K miles. I kinda want to keep my oil changes every 3K, so, I'm not sure if I should just wait until my odometer hits 99K to do my oil change or not.

i no people that only change there oil like once a year. but the car just sit'* in the garage!

Change your oil when ever you want to!
10,000 km or 6,000 miles intervals is not what I would ever do. First off you must run quality synthetic (mobil1 for example) and change the oil filter at the half way point for best results.
Best to have oil analysis done, for safety.

I run full synthetic WIX filter and change my oil every 4000 miles period.
You could ask the dealer why they believe that is safe.
I assume they are obviously using synthetics.

Mobile 1 for sure. a good synthetic will last longer and cost you less money in the long run. I don't think your mileage really matters though.

Normally one year Mike is what the dealer states. However if the car only had 50 miles and then run & stored, hopefully with an ethanol 1 year fuel stabilizer in fuel, as long as there are no contaminants, I still drive for a while once back on the road if the oil is clean & then change it again.
I would say though I would start the vehicle once a week just to keep oil seals and rings lubed. Leaving a car for 6 months and no start is asking for seal problemos.