I have not been inside a 6F35, (not that I'd fit) but most automatic transmissions use clutches (clutch packs that have frictions and steels packaged side-by-side) for direct or 3rd gear operation. The clutch pack engage by hydraulic pressure on a 'piston' (looks like a washer with a large hole in the middle, seals inner and outer edges) that is pushed against springs to squish the frictions and steels together to transfer power. The seals on the piston get hardened with heat and age, and start to leak fluid under pressure, and allow the clutch to 'slip', generating more heat cooking the seals even more... and soon $$$'s learn to fly.
That said, if the clutch for 3rd are slipping, then they are wearing. They are generating heat, and they are slowly cooking the fluid and the transmission. They will not 'self heal' and they won't get better with age or use.
If the unit is under warranty, try taking a spin in a comparable model, to compare shift quality. If yours is bad, get the service writer to take a ride in your vehicle, demonstrate the shift characteristics, the climb into the other vehicle and take the same ride. Denial is more than a river in Egypt. Do not let them deny what is apparent, and if it is truly a slipping transmission, get them to fix it.
Some designs allow slippery clutch apply. So you don't 'feel' the shift. I do not drive those vehicles as I know that slips and slides == heat, and heat is what damages things that are literally bathed in lubricant, such as transmission gizzards. ATF is ALL OVER THE PLACE, flying around under pressure, leaking past seals and valve body pieces, so lube is there. Some parts are shielded from the flow, and some are loaded to they demand 'tubes' to direct flow to keep the cool under load (rear bearings in AXOD), but there IS lube all over.
Most transmissions die from being overheated, and in some cases, part of that is by design. If your bits slip, they ARE heating. The clutch disk in your 5-speed pickup truck gets over 100F within seconds of you backing out of the driveway. Just that little bit of slippage can make it too hot to touch. See also brake shoes, pads, drums and disks. Friction. From things 'sliding' on one another == heat. Period.
tom