It's difficult to accurately predict the weight of a dog at 12 months old based on their weight at 3 weeks old. However, beagles are known to grow rapidly in their first year of life, with most reaching their full adult weight between 9 and 12 months old.
On average, a male beagle will weigh between 20 and 30 pounds at 12 months old. However, the exact weight will depend on a variety of factors, including genetics, diet, and exercise.
It's important to note that puppies require a balanced and nutritious diet to support their growth and development. Consult with a veterinarian to develop a feeding plan that meets the specific needs of your beagle puppy. They can also provide guidance on how to properly care for your puppy as they grow into adulthood.