100% code, ;-)

function footFotPets(input) {
    let days = Number(input[0]);
    let totalFoot = Number(input[1]);
    let sumDogFoot = 0;
    let sumCatFoot = 0;
    let totalEatenFoot = 0;
    let totalPercent = 0;
    let sumBisscuits = 0;
    let totalDogPercents = 0;
    let totalCatPercents = 0;
    let counter = 2;

    for (let i = 1; i <= days; i++) {
        let dogFoot = Number(input[counter++]);
        let catFoot = Number(input[counter++]);

        sumDogFoot += dogFoot;
        sumCatFoot += catFoot;
     
        if (i % 3 == 0) {
            let currentSumBisscuits = (dogFoot + catFoot) * 0.10;
            sumBisscuits += currentSumBisscuits;
        }
    }

    totalEatenFoot = sumDogFoot + sumCatFoot;
    totalDogPercents = (sumDogFoot / totalEatenFoot) * 100;
    totalCatPercents = (sumCatFoot / totalEatenFoot) * 100;
    totalPercent = (totalEatenFoot / totalFoot) * 100;

    console.log(`Total eaten biscuits: ${Math.round(sumBisscuits)}gr.`);
    console.log(`${totalPercent.toFixed(2)}% of the food has been eaten.`);
    console.log(`${totalDogPercents.toFixed(2)}% eaten from the dog.`);
    console.log(`${totalCatPercents.toFixed(2)}% eaten from the cat.`);
}

За 100/100 заменете:

        if (i % 3 === 0){
            buiscuits=0.10*(eatenDog+eatenCat);
 
        }
 
        totalB+=buiscuits;

С този код:

        if (i % 3 === 0){
            totalB+=0.10*(eatenDog+eatenCat);
 
        } 

Проблемът е, че не занулявате променливата buiscuits на всяка итерация, а я прибавяте винаги към totalB, което ще доведе до грешен резултат при повече от 3 дни.