Do i have to go through customs if I fly the same airline on two flights but go to three countries?

You normally do not clear customs as long as you stay within the transit area, but will need to clear security again when you transfer from the arrivals level to the departure level at Suvarnabhumi Airport in Bangkok. The airport here is one giant building and there is no need to transfer between terminals where you would have to clear customs like in some other airports. You will need to clear customs at your final destination. Whether you're on the same airline or not, you usually can stay within the transit area to complete any arrangements if necessary.