Do Steady Breath, Special Fighter and Slaying Axe stack?

Slaying Axe (Slaying/Killer weapons in general) grants a flat (-1) reduction to your special. For example, a 5 count special, like Aether, will always start at 4 when a Slaying weapon is equipped.

Steady Breath only works when an enemy attacks you first. Every hit they give you counts as 2 special counts instead of 1, and your counter attacks also count as 2 instead of 1. So if you have Aether equipped it'll go from 5 to 3 if a foe hits you then from 3 to 1 when you counter attack.

Special Fighter makes it so that your attacks and enemies' attacks count as 2 when you're above 50% health. It also has the added "Guard" effect of preventing enemies from gaining a charge when they hit you. This works in enemy phase and player phase.

Special Fighter and Steady Breath do not stack but you can run a Slaying weapon with either one. Using Special Fighter and Steady Breath together means you'll only get 2 charges when an enemy hits you but you'll still have your guard effect. It doesn't stack up to 4 charges per hit.

No, because only the higher value apply. Both SB and SF had the same value so they work like normal, in the case of Slaying axe it grants you with -1 charge special all the time, and works well with SB or SF but also you can use wrath, quickened pulse, special spiral, infantry pulse and ostia's pulse to reduce more the special activation.