TWOV Question

Yes, you would be eligible for 24 hour visa free transit in the first case because your layover will be shorter than 24 hours. However, you would not be eligible in the second case because your total transit time will be longer than 24 hours.

In any case, having a layover of 3 hours for a self-transfer is relatively risky because you have to be aware that you might lose your onward flight to Jeju. In other words, during your check-in in Dubai you will declare that you will fly to Jeju with a transit in Shanghai that will be covered by 24 hour visa free transit. Here you will present your confirmed ticket on Shanghai to Jeju route. After a short check, you will be allowed to board the plane.

Upon landing the very first thing you should do is to approach visa free transit counter where you will apply for a temporary entry permit by presenting that same flight to Jeju. Again, after a short check you will be granted a permit and then you will head to the immigration in order to formally enter China. After that you need to collect your luggage and to head right to check-in for your flight to Jeju. After that a security and exit immigration and then quickly to the gate for your flight to Jeju.

As you can see, it is doable in 3 hours, but it is also quite tight. It has to be your decision, but I would suggest you to find an option with at least 4 hours of layover.