@ 12 weeks or 3 months I would get the dog out of the yard to do training. I started leash training at 8 weeks old, outside the immediate area's where new thing's could be explored. Gardner's, noises, (horns) smells, cars driving by in the street, kids in baby buggies, ect. Carry some kibble in your pocket or a bag. First have the dog stay in heal on your left, and work from there, it's actually very easy. You should have seen the "perk up" when we both found a "new stick" and she carried it in her mouth as a task, how happy she was to do it.
If it does not happen now, when the dog is 5 to 6 months old he or she will double in strength and then, you have to deal with a dog that can pull a full ox cart, then it becomes difficult, manners now, easy in 2 more months..