What does the owner's manual say for the type driving you do? The OM for my '07 F150 says 5K, I believe it is. My driving is probably ideal, no short trips, warms to operating temperature each trip, no dirty environment, etc and I never let mine go over 5K between changes. If I work the 5.4 hard towing I change the oil more frequently. Motorcraft oil and filter.
The schedule for my E450 V10 recommended 3K changes under severe duty.
If that's the case then the dealer should have changed the oil in my new truck the day I bought it. The build date was 01/11. It's been on the dealers lot since 03/11 and I bought it 06/11.
I think one factor here is that if a truck owned by a person is driven daily for 6 months, for very short distances, it will probably get moisture and some combustion by-products or acids in the oil, and the 6 month change makes sense to me.
If it sits on the dealer lot as pretty much a brand new truck, driven very little, probably less stuff to worry about in the oil. If it was my truck, I'd maybe consider doing the first change a bit early (maybe at 3k miles) and then move to the oil change interval that I would choose to use for the long term, something between 5k and 7.5k miles. But I seriously doubt there would ever be any kind of measurable engine wear no matter what you did.
As for the dealer "making" the OP come in for a change at 3k, many dealers do put their little 3k sticker on the windshield to help you to remember to come in and give them some of your money as often as they can justify If you have it changed observing the intervals in the owner's manual, they cannot deny any warranty work.
If your vehicle is still in warranty...you must follow the manufacterers recommendation to keep the warranty in effect. I believe that recommendations are for time or miles...whichever comes first. Dealers...of course...want you to come in more often...more opportunities to upsell.