Triplanetary Combat Question

[Aman]

Let's say I have a dreadnought, combat 15.
It attacks 3 Corsairs together, combat 4 so 12 Total.
This is a 1:1 attack. I roll a '6' [yay!] for D4 Damage.
How do I allocate damage to the Corsairs?

Is it D4 to each one?
Is it D1 to each plus one randomly gets D2?
Something else?

I can think of plenty of ways to do it that are fair, but is there a rule?
Thanks!