Looking at the Phalanx fighter archetype and saw the Shield Ally ability:
Shield Ally (Ex)
At 9th level, when a phalanx fighter is using a heavy or tower shield, he can, as a move action, provide partial cover (+2 cover bonus to AC, +1 bonus on Reflex saves) to himself and all adjacent allies until the beginning of his next turn.
At 13th level, he can instead provide cover (+4 cover bonus to AC, +2 bonus on Reflex saves) and evasion (as a rogue) to one adjacent ally until the beginning of his next turn. This cover does not allow Stealth checks.
At 17th level, he can provide cover to himself and all adjacent allies, or he can provide improved cover (+8 cover bonus to AC, +4 bonus on Reflex saves, improved evasion)to a single adjacent ally.
This ability replaces weapon training 2, 3, and 4.
My question is as a GM if I throw my players In a 300 type encounter would this ability stack with having 3 of these fighters side by side to a max of 2 bonuses going to the center fighter?
Right but if all three have the shield other feat would that make a difference? I’m trying to design a stressful fight for my players where they realize as a unit if they they to take them, they will be obliterated.
No. The get the bonus of partial cover/cover/improved cover once, as you can receive the same bonus twice.
The middle guy would only get the bonus once, not from both sides. Otherwise, you could have 5 guys and give the middle character 4 sets. Which would be even more ridiculous.
No, its the same bonus and doesn't stack with itself.
"Bonuses without a type always stack, unless they are from the same source." CRB pg. 208
Untyped bonuses stack. Cover, however, does not--a creature can have cover, partial cover, or improved cover against an attack, and that status maps to the defensive bonuses that creature receives.