I suspect this concept has been talked about several times, but i think i might have a new canonicall spin to it so try not to pitchfork me just yet.
I've always been a believer that the Orion was meant to fire three THTs from each of its big turrets, as it says in the table (though i feel it looks better cycling through them one by one rather than in one go, as if each weapon had its own cooldown/recharge cycle and was set to fire while the other two are recharging); i always thought it silly to have that massive turret for a single blob that's only a little bigger than a normal turret. I was already planning to have this on the Terra in the Sol:AH Remake i'm planning (as i'm planning the Levi's in that to have THT's on three of their turrets, so it makes the destroyer rightfully much stronger and let's it be a real centrepoint in battles). As the turrets are so big, it could be said that destroyer emplacements have big enough power reserves and heat sinks to handle multiple heavy weapons like that, but cruisers, and later corvettes, arn't (so while a Deimos's THT turrets would "in reality" have two THT weapons mounted, becuase of power and heat it can only fire one from each barrel at a time at the speed it currently does, represented in the tables by one entry).
While thinking the above over i recently had a thought; of all the situations in Canon FS1 where an Orion was involved, Clash of the Titans was the only one where it really slugged it out with another big ship, and even then bombers were the major factor in that fight. The only other two situations in mind ("La Ruota della Fortuna" when the Galatea takes on the Hellions, and "Tenderizer" when the mauler tries to ram the Galatea), the angles of fighting arn't right to bring the THT turrets into play much and it relied more on the fighters to make the kill. Thus, if the Orion's turrets were made to fire all three, i feel it wouldn't wreck balance in the main campaign a great deal (it might shoot down bombs a lot more often, but most bombing missions start with destroying turrets anyhow). The same could be said in FS2 as the Orion mostly uses its Beam turrets then.
Its not really important (i can have the Terra with it just on its own end of the day), but as i had a canonicall explenation and the table says three guns, i thought i'd get other people's opinions and see what the consensus was on the subject. If we went further with the "destroyer heat sinks" rule, we could even look to the other destroyer and bigger vessels (like the Typhon) later and see if they could be upgraded to match the Orion.
Okay i'm done, you can pitchfork me now.