When atoms are placed at all 12 edges of cube how many atoms are present per unit cell?

Total corners present in a cube = 8

Contribution of 1 atom at corner = $\frac{1}{8}$

Hence 8 atoms will contribute equally towards = $\frac{1}{8}\times 8$ =1

Hence one atom is shared by 4 cube

so contribution of 1 atom

Hence 1 atom is present in 1 unit cell 12 edge-centered atoms ×(1/4)atom per unit cell = 3 atoms.