it also depends, where your final destination is!
check through luggage and domestic transit is only applicable to certain destinations with international airports such as Phuket, Chiang Mai, Chiang Rain, Krabi, Koh Samui and I believe Trat (Koh Chang)

for all other airports you have to go through normal immigration, pick up luggage and check in again; 3 hours should be sufficient time also for that