PFCs have no solution to T3s without major xeno misplay. Ravagers corner camping charge cycling, Crushers being crushers, Usain Bolt Neuro-Machines and boiler gas, there are no PFC tools you can use to counter-play them. You have to hope that xeno T3s do a major misplay in any hopes to kill them.
However, specialists do have tools to reliably kill T3s. B18 can push through corner camping xenos, RPG can punish frontline T3s, Scout is a major DPS source and Snipers can make boilers inefficient. The problem is that you only have 4 specs. These specs can vary in skill due to the popularity of the role and if marines have bald specs then the only way for them to win is if xeno T3s are also bald.
Another problem is that xenos can gain new T3s, while a lost spec is gone forever. Not to mention if the spec loses their gun or for some reason runs out of ammo. Xeno T3s slots scale due to population while marines are stuck with a static 4 per round.
If marines are going to be spec dependent in taking down T3s, then spec slots should also scale from population. And I don't care about that one time you killed a T3 as a PFC. The Xeno misplayed, you didn't do anything special.