Its logical to do one wall at a time but it doesn t work out that way.
Tiling walls that are not plumb.
If you re tiling directly onto the wall however leveling clips and wedges will force the edges of the.
The horizontal line assures the 3 walls are level with each other and the grout joints will line up wall to wall.
Answer answer what should have been done while installing the hardibacker board on the walls is the framing studs should have been shimmed and adjusted to make sure the hardibacker board was plumb and square.
Subway tiles are notorious for not being well made so i always tile all 3 walls at the same time so the grout joints match up on adjoining walls.
Do this prior to any tiling job.
Check the wall first to see if it is straight.
If the walls were not too far out of wack you can use the thin set mortar or there are special patching mortars for going over the hardibacker board as long as it isn t applied too thick.