I've been to lots of high tech, high population systems looking for 3 gimballed multi-cannon. Where can I find them in Empire space (without a permit, as my Empire ascension is bugged at the moment)? Do they exist? If not do we have any confirmation from the devs that they're not supposed to (i.e. they aren't just really rare).
There are no class 3 (large) multi-cannons, railguns, missile racks, torpedo pylons, mine launchers, or mining lasers.
I believe that's because a class 3 multi-cannon would be too powerful and would unbalance things. Same with some of the other weapons lacking a class 3 variant. (Though some might lack a class 3 variant because it seemed unlikely to be useful).
For class 4 (huge), there are only cannon and plasma accelerator.