How can the allele be recessive when three children have the disease ? (the answer is c )

Both parents are normal. But, some of the kids have the disease. This indicates that the parents are heterozygous for the disease (let's use Dd). They are phenotypically fine because D is normal. But we know they must both carry the diseased d allele because their kids have the disease.
If you cross Dd x Dd you get DD, Dd, Dd and dd as the possible outcomes. But because each time they have a child they roll the dice as to which of the 4 above happens, they were unlucky as 3 of those times they got dd for their child...so 3 of them have the disease (and are dd). One is phenotypically normal but could still be Dd or DD.