depends on the flash, some of the yongnuo flashes have YN622 triggers built in (i think?). i have one yn622TX to trigger, and 4 yn622 as receiver (you connect them with the flash you wanna trigger).
so, if you want to trigger 3 external flashes, you need either 4 YN622 (where one of them acts as trigger on your cam) or 1 YN622Tx on your cam and 3 YN622s on your flashes.