3 hours normally should be enough in BKK. Domestic and International are in the same building, just lots of walking.
But keep in mind: If your first flight is delayed and you miss the check in deadline there is no obligation for the second carrier to transport you and depending on the type of ticket you might need to buy a completely new ticket. The risk is obviously far bigger on the return, so you might want to move your night in BKK to the return.