Transmission 3rd gear Slipping

Getting it out will be a project, ESPECIALLY with an FF layout. You need to remove the engine to remove the tranny, which requires access to an engine hoist, as well as bumper and radiator removal on a lot of Hyundai chassis. You'll also need to remove the front driveshaft with FWD vehicles to pull the motor unless the bumper and radiator suffice. It's easier on an FR layout (or 4WD/AWD since they usually use an FR layout) since the driveshaft has to come out, but it's in line with the rear wheels, making removing it many degrees easier.

FR=Front engine, rear drive/FF=Front engine, front drive

I would never rebuild a tranny these days, especially with the lifetime fluid bullshit the OEMs push. The lifetime is the mechanical warranty. Once issues like gear slip come up, they're hiding other major problems way too often to the point a complete rebuild needs to be done. This has become the norm that much these days with how often dealers claim the fluid never needs changing (you're supposed to do it every 50k miles!!!). It's just way too common of an occurrence on non-Aisin transmissions where it's never been done unless the car has 200k miles.

You can't even drain them without risking a tranny failure if you drain the "dirty fluid" from them because THAT IS KEEPING IT ALIVE! If you have to pull it you can't use new fluid -- you're reusing the dirty fluid that kept it alive.

Buy a good reman tranny from Kia/Hyundai, or try your luck at a junkyard. You can't rebuild modern trannies that easily these days, and when they get to this point, there are almost always other issues waiting to greet you. It may end up totaling the vehicle out, depending on the resale. A lot of those 2011-15 Kia/Hyudais have the "100k mile resale" penalty, which makes big jobs sometimes cost-prohibitive. Something small is fair game, but not a big job like a bad tranny.