Kirby has the ability to inhale and store food in his stomach, which he can then use to shoot out as a powerful attack. This gives him a significant advantage over the other two contestants, who do not have the same ability.
Goku
Goku is a Saiyan, and Saiyans are known for their voracious appetites. However, Goku is not the largest of Saiyans, and he does not have the same ability as Kirby to store food in his stomach. This puts him at a disadvantage in an eating contest.
The Flash
The Flash is the fastest man alive, but speed does not necessarily translate to eating ability. In fact, the Flash's high metabolism may actually work against him in an eating contest, as he would need to consume more food to maintain his energy levels.
Conclusion
Based on these factors, Kirby is the most likely winner of an eating contest between Goku and the Flash.