3 depth walking help

Hello everybody,

I’m creating a side scroller, and I came with a problem, i want to make my character go only in 3 directions, like showed in a picture. How I should do this? Can anyone help me with this?

I would make it so when one of the controller keys is pressed (up or down i assume) the character moves forward or backward depending on which button is pressed the exact same distance of one of the lanes. So say one lane is 1 meter wide and the character was in the centre of the middle lane running forward and wants to move to the bottom lane, the player would press the down arrow key, and the character would move to its right by exactly 1 meter, leaving it in the centre of the bottom lane.