Hi Chap,
I've gone back to Mongoose to clarify.
My reading into the rules is that on Full Auto the ACR with auto/3 would use 9 rounds per attack. Full Auto is not about conserving ammo.
So for ACR, single shot = 1 round per attack, burst = 3 rounds per attack and full auto = 9 rounds per attack.
Soon as I have clarification I'll post here and update the ruleset.
*** EDIT ***
Mongoose have replied:
In this example, you would get three separate attacks, and expend 9 rounds of ammo.
So I'll double check all is good with the above.
Cheers,
MBM