Sorry for delay replying. Likely yes; you will want to use one of those flags on any turret you want using many guns.
If you want all three guns firing at the same time, then just adding the 'salvo mode' to what you have will do it (it'll take the turret's one weapon and assume every fire-point of the turret as that weapon mounted).
If you want them firing separately however you'll likely want to use one of two methods (depending on how you want it to behave);
You can have multiple weapons in the entry (entered in the brackets with a space between each of them) and then add two flags, "use multiple guns" (which tells the turret to use all the weapons in it's entry; standard turrets will only use the first weapon AFAIK) then "fixed firing points" (this tells the system to lock each weapon to a specific fire point; weapon 1 will only ever use point 1, weapon 2 point 2, etc). The weapons would fire at their own speed, so it'd be like several independent guns on one turret. You'll likely want as many weapons as there are barrels in this case (so three mass drivers, for example).
Another option is a bit more fiddly to enter, but allows you to have the turret steadily cycling through the guns so it can fire the weapon faster than it's normal ROF rate (as if the gunners were firing one, then another while waiting for the first to reload, and so on). For that you have just one weapon entry, put in the "use multiple guns" (which tells it to cycle through the firing points when it's firing), and then add an "$ROF:" tag to the turret with the "+Use firingpoints" tag, which tells it to multiply the ROF by the number of barrels.
If you go with that second option, the $ROF tags go AFAIK just before the "$flags" tag, the '+use firingpoints' below '$ROF' (the system is a bit strict about how things should be ordered), so the turret entry should end up looking something like this;
$Subsystem: Turret03, 4.0, 3.0
$Default PBanks: ( "Mass Driver#karuna" )
$ROF:
+Use firingpoints
$Flags: ( "check hull" "use multiple guns" )
As for the mission loading and the texture, I'm not entirely up to speed on that I'm afraid but it is possible (I recall having similar long load times with the Erebus, which also uses large textures). Might just be a downside to quality models; The high-poly Typhon is infamous for that too apparently. Normally such long load times is only the first few loads; after that it's in cache and so should go a bit faster hopefully.
Hope that helps. Really looking forward to this as it's a cool looking ship. Nice turret placement too
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