Unless you operate in exceptionally adverse conditions no way you need to change it every 3K mi and I don't see how a dealer can override the service manual. But, . . it's your car.
While we're on the subject, KIA uses the same tranny fluid as Hyundai so read & heed: "Is ATF+3 OK TO USE IN MY SONATA, because i was told it had to be purchased from dealer only? so can i just use atf+3 from the locale parts shop, and still be ok in warranty?
If you want to buy a new transmission, go right ahead. I have heard that ATF+3 will get you buy if you are just topping off, but not for a drain and fill. (emphasis that "I have heard" but have not seen, so don't mark my words) I have sold several transmissions to shops that put in Dextron 3 or ATF3. Some have not even made it out of the parking lot. If you are that cocerned about price, check for the SPIII fluid at a Kia or Mitsubishi dealership, and check their prices, it is the same fluid for all 3 car lines. One day an aftermarket company might make this fluid, but I doubt it will be anytime soon, since Hyundai has put this fluid in from the beginning. Be forwarned though, if you do put in ATF3, and your car does run for awhile, but your transmission goes out in the future, you will be buying that transmission. When we send the cores back to Hyundai, they do test the fluid that is in it for any contaminents or any foreign fluids or additives and it will come back on you.
I have seen it happen. Just spend the extra money and buy the SPIII, might save you a lot in the future."
Also worth of note, the power steering fluid is unique so get it from KIA. Please tell me how much they charged for oil & filter change.
Very good info in this link: Everything you need to know about engine oil