Each hectare of Johnson's neighbor can yield 80 x 2 = <<80*2=160>>160 corns every two months.
So, the neighbor's two-hectare cornfield can yield a total of 160 x 2 = <<160*2=320>>320 corns every two months.
They yield a total of 80 + 320 = <<80+320=400>>400 corns.
Therefore, They can harvest 400 x 3 = <<400*3=1200>>1200 corns altogether.
#### 1200