Imho, each vehicle should fire separatly, if the 3 tanks are in a compagny (contains 3 detachments of 1 tank and 1 is a CHQ) as all SHT IG company

We should also add:
"declare at the start of each turn which weapon is used"

Also about missiles choices, it could be a solution to try to compare them with titan weapons:

- titan MRL cost 75 points with a 12cm shotgun mode close to the firestorm missile
- titan AA cerberus cost 40 points with a AA gun close to the profile I proposed, bringing it to 3 attack dices and you have a 60 point pillum missile
- titan volcano cannon cost 100 points, by reducing to hit, TSM and penetrating values by one point and you should get a 75-60 points foehammer missile

This way, you'd get 3 missiles that should be balanced in cost and power

Imperial Preator heavy assault launcher:

Move 10cm
Armour 2+
CAF +3

Weapons:
Heavy bolters
Attack dice 2
Range 25cm
to hit 5+
TSM +0

Praetor heavy assault launcher:
may fire one of the following per turn:

Foehammer missiles:
Attack dice 2
Range 100cm
to hit 5+
TSM -3
penetrates +1

Firestorm missiles
Attack dice 6BP
Range 150cm
to hit 6BP
TSM -1


Pilum AA-missile system
Attack dice 5
Range 100cm
to hit 6+
TSM -2
ignore any "to hit" modifers
Notes: super heavy
Morale Value 4+
PD(4)

Declare at the start opf each turn which weapon is used

No Problem with that
I oriented after the WH40K wiki site

but with 2 detachments you can transport a det. Space Marines

But with 2 detachments, you're not allowed to transport 1 infantry det

About the cost, I usually prefer playing with the cost too than trying to get all variants the same cost. If a tank is better, it should just cost more.