Within a cubic unit cell, sketch the directions of [012], [721], [110].

[012]:

Here x component is zero. That is why this is in y - z plane

[721]:

This is in all three plane. It means it is in space.

[110]:

Here z component is zero.

That is why this is in x - y plane

From the cubic unit directions we can easily understand all given directions [012], [721], [110].