In the scenario described, a drought wipes out the plants that both Bird A and Bird C rely on for food (plant seeds). Bird A eats insects and plant seeds, and Bird C only eats plant seeds. With the disappearance of plant seeds, Bird A's food source is compromised, but it may still survive if there are enough insects available since it can eat them as well. Bird C, however, relies solely on plant seeds and will not survive without them.
Bird B drinks flower nectar, so it could survive if there are any remaining flowers or if it can find alternative sources of food.
Thus, the most likely outcome is:
Bird A survives; either bird B or bird C dies, and the other survives.
This is because Bird A can consume insects while Bird C will likely die due to the loss of plant seeds. Bird B's survival will depend on whether it can find any remaining sources of nectar.