In MGT High Guard is stated particle accelerators may only be mounted as single weapons, but they need a triple turret to be mounted (LBB2, page 47).
My surprise was when I checked again the core book (page 111) and I found that, even as PA turret weapon is TL 8 (quite earlier than CT or MT, where they appeared at TL 14), triple turret is TL 9. SO, how can I mount a PA on a TL 8 ship? How could they develop a weapon they could not mount on the ship? Or there's another way to mount it, aside from a triple turet?
You are working too hard at this.
At TL 6, a civilization can build a normal sized Missile Rack. There are no Starship Turrets available, so Fixed Mounting the Missile Rack to a Hardpoint is the only option available.
At TL 7, a civilization can build a normal sized Beam Laser, Pulse Laser and Sandcaster (plus the TL 6 Missile Rack). The civilization also gains the ability to mount one weapon per turret to a starship hardpoint (the TL 7 Single Turret). Since Single Turrets are the only ones available, only one normal sized weapon (a Missile Rack, Beam Laser, Pulse Laser or Sandcaster) can be mounted or swapped interchangeably in the single normal sized weapon bracket found inside a normal Single Turret.
At TL 8, a civilization gains the ability to mount two normal sized weapons (a Missile Rack, Beam Laser, Pulse Laser or Sandcaster) in a larger turret to a starship hardpoint (the TL 8 Double Turret). This does not simply represent the ability to build a larger turret, but includes more advanced fire control to allow two weapons to target the same point and converge fire or allow two weapons to target different objects and coordinate fire to allow each weapon to hit its target with no loss of accuracy. Since both Single Turrets and Double Turrets are available, one normal sized weapon can be mounted in the single normal sized weapon bracket found inside a normal Single Turret or two normal sized weapons can be mounted in the two normal sized weapon brackets found inside a normal Double Turret.
At TL 9, a civilization gains the ability to mount three normal sized weapons (a Missile Rack, Beam Laser, Pulse Laser or Sandcaster) in a still larger turret to a starship hardpoint (the TL 9 Triple Turret). This does not simply represent the ability to build a larger turret, but includes more advanced fire control to allow three weapons to target the same point and converge fire or allow each weapon to target different objects and coordinate fire to allow all weapons to hit their target with no loss of accuracy. Since the Single Turret, Double Turret and Triple Turret are available, one normal sized weapon can be mounted in the single normal sized weapon bracket found inside a normal Single Turret or two normal sized weapons can be mounted in the two normal sized weapon brackets found inside a normal Double Turret or three normal sized weapons can be mounted in the three normal sized weapon brackets found inside a normal Triple Turret.
Now for the Special Case.
At TL 8, some weapon scientist with serious inadequacy issues builds “The Mother Of All Weapons” – a Particle Accelerator. One small problem, however, is that the PA is as large as THREE normal sized weapons. Fortunately, we are in luck, because the equipment needed to mount a weapon in a turret and aim it at a single target was perfected back at TL 7. That said, the PA will not fit in a normal sized weapon bracket, so we need to build a super-sized bracket and a super-sized fire control for our super-sized weapon (the TL 8 turret-mounted Particle Accelerator). All of this super-sized equipment means that a Particle Accelerator Giant Single Turret is as large as a TL 9 Triple Turret for normal sized weapons. As a Single Turret, the PA Turret could be built at TL 7 (although the weapon could not), and because of its size, the Giant PA Single Turret costs as much as a Triple Turret for normal weapons.
Does that make sense?