To determine the phenotype of the offspring indicated by box 3 in the Punnett square, we really need to see the alleles on the indicated box. However, since the focus is on the dominance of the alleles, we can infer the following:
Black fur (B) is dominant to brown fur (b). Therefore, any offspring that has at least one B allele will have black fur.
If you provide the alleles that correspond to box 3 in the Punnett square (for example, BB, Bb, or bb), I can give you the specific phenotype. However, if box 3 contains at least one B, the phenotype will be black fur. If it has two b alleles (bb), then the phenotype will be brown fur.
Assuming box 3 contains either BB or Bb, the phenotype would be black. If it contains bb, the phenotype would be brown.
Based on common Punnett square outcomes from B and b crosses, please confirm the alleles in that box for a more precise answer.